diff --git a/Arrangement_on_surface_2/doc/Arrangement_on_surface_2/Arrangement_on_surface_2.txt b/Arrangement_on_surface_2/doc/Arrangement_on_surface_2/Arrangement_on_surface_2.txt
index 7c143578c89..4d95b4e71d3 100644
--- a/Arrangement_on_surface_2/doc/Arrangement_on_surface_2/Arrangement_on_surface_2.txt
+++ b/Arrangement_on_surface_2/doc/Arrangement_on_surface_2/Arrangement_on_surface_2.txt
@@ -5706,17 +5706,19 @@ changes when they occur. In our case observers can be attached to an
 arrangement object. An attached observer receives notifications about
 the changes this arrangement undergoes.
 
-An observer object, the type of which is an instance of the
-`Arr_observer<Arrangement>` class template, stores a pointer to an
-arrangement object. When the `Arr_observer<Arrangement>` class
-template is instantiated, the `Arrangement` parameter must be
-substituted with the type of the arrangement object. The observer
-receives notifications <em>just before</em> a structural change occurs
-in the arrangement and <em>immediately after</em> such a change takes
-place. `Arr_observer` serves as a base class for other
-observer classes and defines a set of virtual notification
-functions, with default empty implementations. The set of functions
-can be divided into three categories, as follows:
+An observer object that observes changes that an arrangement object of
+type `Arrangement` undergoes must be of a type derived from
+`Arrangement::Observer`. The observer object stores a pointer to the
+observed arrangement object. The observer receives notifications
+<em>just before</em> a structural change occurs in the arrangement and
+<em>immediately after</em> such a change takes place.
+`Arrangement::Observer` is an alias to
+`Arr_observer<CGAL::Arrangement_on_surface_2<>>`, where
+`CGAL::Arrangement_on_surface_2<>` is the type of the arrangement
+object or its base type. It serves as a base type for other observer
+classes and defines a set of virtual notification functions, with
+default empty implementations. The set of functions can be divided
+into three categories, as follows:
 
 <OL>
 
diff --git a/Arrangement_on_surface_2/doc/Arrangement_on_surface_2/CGAL/Arr_face_index_map.h b/Arrangement_on_surface_2/doc/Arrangement_on_surface_2/CGAL/Arr_face_index_map.h
index bcb0d5c378f..4c48eb10207 100644
--- a/Arrangement_on_surface_2/doc/Arrangement_on_surface_2/CGAL/Arr_face_index_map.h
+++ b/Arrangement_on_surface_2/doc/Arrangement_on_surface_2/CGAL/Arr_face_index_map.h
@@ -21,12 +21,11 @@ to dynamically maintain the mapping of face handles to indices.
 
 \cgalModels{DefaultConstructible,CopyConstructible,Assignable,ReadablePropertyMap}
 
-\sa `Arr_observer<Arrangement>`
 \sa `Arr_vertex_index_map<Arrangement>`
 */
 
 template< typename Arrangement >
-class Arr_face_index_map: public Arr_observer<Arrangement> {
+class Arr_face_index_map: public Arrangement::Observer {
 public:
 
 /// \name Types
@@ -74,4 +73,3 @@ Arr_face_index_map(Arrangement_2& arr);
 
 }; /* end Arr_accessor */
 } /* end namespace CGAL */
-
diff --git a/Arrangement_on_surface_2/doc/Arrangement_on_surface_2/CGAL/Arr_triangulation_point_location.h b/Arrangement_on_surface_2/doc/Arrangement_on_surface_2/CGAL/Arr_triangulation_point_location.h
index 5106f32ec36..2bd83d5a125 100644
--- a/Arrangement_on_surface_2/doc/Arrangement_on_surface_2/CGAL/Arr_triangulation_point_location.h
+++ b/Arrangement_on_surface_2/doc/Arrangement_on_surface_2/CGAL/Arr_triangulation_point_location.h
@@ -23,7 +23,6 @@ namespace CGAL {
  */
 
 template <typename Arrangement_>
-class Arr_triangulation_point_location : public Arr_observer<Arrangement_>
-{}
+class Arr_triangulation_point_location : public Arrangement_::Observer {}
 
 }
diff --git a/Arrangement_on_surface_2/doc/Arrangement_on_surface_2/CGAL/Arr_vertex_index_map.h b/Arrangement_on_surface_2/doc/Arrangement_on_surface_2/CGAL/Arr_vertex_index_map.h
index 1c915a6b23c..af17eafb8a7 100644
--- a/Arrangement_on_surface_2/doc/Arrangement_on_surface_2/CGAL/Arr_vertex_index_map.h
+++ b/Arrangement_on_surface_2/doc/Arrangement_on_surface_2/CGAL/Arr_vertex_index_map.h
@@ -21,12 +21,11 @@ to dynamically maintain the mapping of vertex handles to indices.
 
 \cgalModels{DefaultConstructible,CopyConstructible,Assignable,ReadablePropertyMap}
 
-\sa `Arr_observer<Arrangement>`
 \sa `Arr_face_index_map<Arrangement>`
 */
 
 template< typename Arrangement >
-class Arr_vertex_index_map: public Arr_observer<Arrangement> {
+class Arr_vertex_index_map: public Arrangement::Observer {
 public:
 
 /// \name Types
@@ -74,9 +73,3 @@ Arr_vertex_index_map(Arrangement_2& arr);
 
 }; /* end Arr_accessor */
 } /* end namespace CGAL */
-
-
-
-
-
-
diff --git a/Arrangement_on_surface_2/examples/Arrangement_on_surface_2/face_extension.cpp b/Arrangement_on_surface_2/examples/Arrangement_on_surface_2/face_extension.cpp
index 8befee7dafc..21494ebb891 100644
--- a/Arrangement_on_surface_2/examples/Arrangement_on_surface_2/face_extension.cpp
+++ b/Arrangement_on_surface_2/examples/Arrangement_on_surface_2/face_extension.cpp
@@ -3,7 +3,6 @@
 
 #include <CGAL/basic.h>
 #include <CGAL/Arr_extended_dcel.h>
-#include <CGAL/Arr_observer.h>
 
 #include "arr_exact_construction_segments.h"
 
@@ -12,15 +11,16 @@ using Ex_arrangement = CGAL::Arrangement_2<Traits, Dcel>;
 
 // An arrangement observer, used to receive notifications of face splits and
 // to update the indices of the newly created faces.
-class Face_index_observer : public CGAL::Arr_observer<Ex_arrangement> {
+class Face_index_observer : public Ex_arrangement::Observer {
 private:
   size_t n_faces;                       // the current number of faces
 
 public:
   Face_index_observer(Ex_arrangement& arr) :
-    CGAL::Arr_observer<Ex_arrangement>(arr), n_faces(0)
+    Ex_arrangement::Observer(arr),
+    n_faces(0)
   {
-    CGAL_precondition (arr.is_empty());
+    CGAL_precondition(arr.is_empty());
     arr.unbounded_face()->set_data (0);
   }
 
diff --git a/Arrangement_on_surface_2/examples/Arrangement_on_surface_2/observer.cpp b/Arrangement_on_surface_2/examples/Arrangement_on_surface_2/observer.cpp
index d4261d64dd2..547fdab09c0 100644
--- a/Arrangement_on_surface_2/examples/Arrangement_on_surface_2/observer.cpp
+++ b/Arrangement_on_surface_2/examples/Arrangement_on_surface_2/observer.cpp
@@ -2,15 +2,14 @@
 // Using a simple arrangement observer.
 
 #include <CGAL/basic.h>
-#include <CGAL/Arr_observer.h>
 
 #include "arr_exact_construction_segments.h"
 #include "arr_print.h"
 
 // An observer that receives notifications of face splits and face mergers.
-class My_observer : public CGAL::Arr_observer<Arrangement> {
+class My_observer : public Arrangement::Observer {
 public:
-  My_observer(Arrangement& arr) : CGAL::Arr_observer<Arrangement>(arr) {}
+  My_observer(Arrangement& arr) : Arrangement::Observer(arr) {}
 
   virtual void before_split_face(Face_handle, Halfedge_handle e) {
     std::cout << "-> The insertion of :  [ " << e->curve()
diff --git a/Arrangement_on_surface_2/include/CGAL/Arr_face_index_map.h b/Arrangement_on_surface_2/include/CGAL/Arr_face_index_map.h
index 1e03af5e1c6..3fb3915b65a 100644
--- a/Arrangement_on_surface_2/include/CGAL/Arr_face_index_map.h
+++ b/Arrangement_on_surface_2/include/CGAL/Arr_face_index_map.h
@@ -21,7 +21,6 @@
  * Definition of the Arr_face_index_map<Arrangement> class.
  */
 
-#include <CGAL/Arr_observer.h>
 #include <CGAL/Unique_hash_map.h>
 #include <CGAL/property_map.h>
 
@@ -35,8 +34,7 @@ namespace CGAL {
  * of faces in the arrangement.
  */
 template <class Arrangement_>
-class Arr_face_index_map : public Arr_observer<Arrangement_>
-{
+class Arr_face_index_map : public Arrangement_::Observer {
 public:
 
   typedef Arrangement_                            Arrangement_2;
@@ -51,7 +49,7 @@ public:
 private:
 
   typedef Arr_face_index_map<Arrangement_2>       Self;
-  typedef Arr_observer<Arrangement_2>             Base;
+  typedef typename Arrangement_2::Observer        Base;
 
   typedef Unique_hash_map<Face_handle, unsigned int>     Index_map;
 
@@ -73,14 +71,14 @@ public:
 
   /*! Constructor with an associated arrangement. */
   Arr_face_index_map (const Arrangement_2& arr) :
-    Base (const_cast<Arrangement_2&> (arr))
+    Base(const_cast<Arrangement_2&>(arr))
   {
     _init();
   }
 
   /*! Copy constructor. */
   Arr_face_index_map (const Self& other) :
-    Base (const_cast<Arrangement_2&> (*(other.arrangement())))
+    Base(const_cast<typename Base::Arrangement_2&>(*(other.arrangement())))
   {
     _init();
   }
@@ -92,7 +90,7 @@ public:
       return (*this);
 
     this->detach();
-    this->attach (const_cast<Arrangement_2&> (*(other.arrangement())));
+    this->attach(const_cast<typename Base::Arrangement_2&>(*(other.arrangement())));
 
     return (*this);
   }
diff --git a/Arrangement_on_surface_2/include/CGAL/Arr_point_location/Arr_lm_generator_base.h b/Arrangement_on_surface_2/include/CGAL/Arr_point_location/Arr_lm_generator_base.h
index daf08604ed2..07cf7520a13 100644
--- a/Arrangement_on_surface_2/include/CGAL/Arr_point_location/Arr_lm_generator_base.h
+++ b/Arrangement_on_surface_2/include/CGAL/Arr_point_location/Arr_lm_generator_base.h
@@ -20,7 +20,6 @@
 * Definition of the Arr_landmarks_generator_base<Arrangement> template.
 */
 #include <CGAL/Arr_point_location_result.h>
-#include <CGAL/Arr_observer.h>
 #include <CGAL/Arrangement_2/Arr_traits_adaptor_2.h>
 #include <CGAL/Arr_point_location/Arr_lm_nearest_neighbor.h>
 #include <CGAL/Arr_batched_point_location.h>
@@ -43,8 +42,8 @@ namespace CGAL {
 */
 template <typename Arrangement_,
           typename Nearest_neighbor_ =
-            Arr_landmarks_nearest_neighbor <Arrangement_> >
-class Arr_landmarks_generator_base : public Arr_observer <Arrangement_> {
+            Arr_landmarks_nearest_neighbor<Arrangement_> >
+class Arr_landmarks_generator_base : public Arrangement_::Observer {
 public:
   typedef Arrangement_                                Arrangement_2;
   typedef Nearest_neighbor_                           Nearest_neighbor;
@@ -110,7 +109,7 @@ public:
    * \param arr (in) The arrangement.
    */
   Arr_landmarks_generator_base(const Arrangement_2& arr) :
-    Arr_observer<Arrangement_2> (const_cast<Arrangement_2 &>(arr)),
+    Arrangement_2::Observer(const_cast<Arrangement_2&>(arr)),
     m_traits(static_cast<const Traits_adaptor_2*>(arr.geometry_traits())),
     m_ignore_notifications(false),
     m_ignore_remove_edge(false),
diff --git a/Arrangement_on_surface_2/include/CGAL/Arr_point_location/Arr_lm_vertices_generator.h b/Arrangement_on_surface_2/include/CGAL/Arr_point_location/Arr_lm_vertices_generator.h
index 57478ec7936..01fd5aba052 100644
--- a/Arrangement_on_surface_2/include/CGAL/Arr_point_location/Arr_lm_vertices_generator.h
+++ b/Arrangement_on_surface_2/include/CGAL/Arr_point_location/Arr_lm_vertices_generator.h
@@ -98,14 +98,13 @@ public:
   virtual void build_landmark_set()
   {
     // Go over the arrangement, and insert all its vertices as landmarks.
-    NN_Point_list         nnp_list;
-    const Arrangement_2*  arr = this->arrangement();
-    Vertex_const_iterator vit;
+    NN_Point_list nnp_list;
+    const auto* arr = this->arrangement();
     num_landmarks = 0;
-    for (vit = arr->vertices_begin(); vit != arr->vertices_end(); ++vit) {
+    for (auto vit = arr->vertices_begin(); vit != arr->vertices_end(); ++vit) {
       Vertex_const_handle vh = vit;
       nnp_list.push_back(NN_Point_2(vh->point(), this->pl_make_result(vh)));
-      num_landmarks++;
+      ++num_landmarks;
     }
 
     // Update the search structure.
diff --git a/Arrangement_on_surface_2/include/CGAL/Arr_trapezoid_ric_point_location.h b/Arrangement_on_surface_2/include/CGAL/Arr_trapezoid_ric_point_location.h
index 7a4042e9601..d04136967ba 100644
--- a/Arrangement_on_surface_2/include/CGAL/Arr_trapezoid_ric_point_location.h
+++ b/Arrangement_on_surface_2/include/CGAL/Arr_trapezoid_ric_point_location.h
@@ -26,7 +26,6 @@
 #include <CGAL/Arrangement_2/Arr_traits_adaptor_2.h>
 #include <CGAL/Arr_point_location/Trapezoidal_decomposition_2.h>
 #include <CGAL/Arr_point_location/Td_traits.h>
-#include <CGAL/Arr_observer.h>
 
 namespace CGAL {
 
@@ -36,7 +35,7 @@ namespace CGAL {
  * The Arrangement parameter corresponds to an arrangement instantiation.
  */
 template <typename Arrangement_>
-class Arr_trapezoid_ric_point_location : public Arr_observer <Arrangement_> {
+class Arr_trapezoid_ric_point_location : public Arrangement_::Observer {
 public:
   //type of arrangement on surface
   typedef Arrangement_                          Arrangement_on_surface_2;
@@ -148,8 +147,8 @@ public:
                            bool with_guarantees = true,
                            double depth_thrs = CGAL_TD_DEFAULT_DEPTH_THRESHOLD,
                            double size_thrs = CGAL_TD_DEFAULT_SIZE_THRESHOLD) :
-    Arr_observer<Arrangement_on_surface_2>
-              (const_cast<Arrangement_on_surface_2 &>(arr)),
+    Arrangement_on_surface_2::
+    Observer(const_cast<Arrangement_on_surface_2&>(arr)),
     m_with_guarantees(with_guarantees)
   {
     m_traits = static_cast<const Traits_adaptor_2*> (arr.geometry_traits());
@@ -352,7 +351,7 @@ protected:
     std::vector<Halfedge_const_handle> he_container;
     Edge_const_iterator eit;
     Halfedge_const_handle he_cst;
-    Arrangement_on_surface_2 *arr = this->arrangement();
+    auto* arr = this->arrangement();
     //collect the arrangement halfedges
     for (eit = arr->edges_begin(); eit != arr->edges_end(); ++eit)
     {
diff --git a/Arrangement_on_surface_2/include/CGAL/Arr_triangulation_point_location.h b/Arrangement_on_surface_2/include/CGAL/Arr_triangulation_point_location.h
index e3f5c3565ff..98e0f1742fd 100644
--- a/Arrangement_on_surface_2/include/CGAL/Arr_triangulation_point_location.h
+++ b/Arrangement_on_surface_2/include/CGAL/Arr_triangulation_point_location.h
@@ -39,8 +39,7 @@ namespace CGAL {
  * triangulation algorithm.
  */
 template <typename Arrangement_>
-class Arr_triangulation_point_location : public Arr_observer<Arrangement_>
-{
+class Arr_triangulation_point_location : public Arrangement_::Observer {
 public:
   typedef Arrangement_                                  Arrangement_2;
 
@@ -129,7 +128,7 @@ public:
    * \param arr (in) The arrangement.
    */
   Arr_triangulation_point_location(const Arrangement_2& arr) :
-    Arr_observer<Arrangement_2>(const_cast<Arrangement_2&>(arr)),
+    Arrangement_2::Observer(const_cast<Arrangement_2&>(arr)),
     m_traits(static_cast<const Traits_adaptor_2*>(arr.geometry_traits())),
     m_ignore_notifications(false),
     m_ignore_remove_edge(false)
diff --git a/Arrangement_on_surface_2/include/CGAL/Arr_vertex_index_map.h b/Arrangement_on_surface_2/include/CGAL/Arr_vertex_index_map.h
index ac6902cc9e2..92f412d2495 100644
--- a/Arrangement_on_surface_2/include/CGAL/Arr_vertex_index_map.h
+++ b/Arrangement_on_surface_2/include/CGAL/Arr_vertex_index_map.h
@@ -22,7 +22,6 @@
 /*! \file
  * Definition of the Arr_vertex_index_map<Arrangement> class.
  */
-#include <CGAL/Arr_observer.h>
 #include <CGAL/Unique_hash_map.h>
 #include <CGAL/property_map.h>
 
@@ -36,8 +35,7 @@ namespace CGAL {
  * of vertices in the arrangement.
  */
 template <class Arrangement_>
-class Arr_vertex_index_map : public Arr_observer<Arrangement_>
-{
+class Arr_vertex_index_map : public Arrangement_::Observer {
 public:
 
   typedef Arrangement_                            Arrangement_2;
@@ -52,7 +50,7 @@ public:
 private:
 
   typedef Arr_vertex_index_map<Arrangement_2>     Self;
-  typedef Arr_observer<Arrangement_2>             Base;
+  typedef typename Arrangement_2::Observer        Base;
 
   typedef Unique_hash_map<Vertex_handle, unsigned int>     Index_map;
 
@@ -67,21 +65,21 @@ public:
 
   /*! Default constructor. */
   Arr_vertex_index_map () :
-    Base (),
+    Base(),
     n_vertices (0),
     rev_map (MIN_REV_MAP_SIZE)
   {}
 
   /*! Constructor with an associated arrangement. */
   Arr_vertex_index_map (const Arrangement_2& arr) :
-    Base (const_cast<Arrangement_2&> (arr))
+    Base(const_cast<Arrangement_2&>(arr))
   {
     _init();
   }
 
   /*! Copy constructor. */
   Arr_vertex_index_map (const Self& other) :
-    Base (const_cast<Arrangement_2&> (*(other.arrangement())))
+    Base(const_cast<typename Base::Arrangement_2&> (*(other.arrangement())))
   {
     _init();
   }
@@ -93,7 +91,7 @@ public:
       return (*this);
 
     this->detach();
-    this->attach (const_cast<Arrangement_2&> (*(other.arrangement())));
+    this->attach (const_cast<typename Base::Arrangement_2&> (*(other.arrangement())));
 
     return (*this);
   }
diff --git a/Arrangement_on_surface_2/include/CGAL/Arrangement_2.h b/Arrangement_on_surface_2/include/CGAL/Arrangement_2.h
index 5ff66d78b30..1861d50cbae 100644
--- a/Arrangement_on_surface_2/include/CGAL/Arrangement_2.h
+++ b/Arrangement_on_surface_2/include/CGAL/Arrangement_2.h
@@ -115,12 +115,9 @@ public:
   typedef typename Base::Inner_ccb_const_iterator  Hole_const_iterator;
 
 private:
-
-  friend class Arr_observer<Self>;
   friend class Arr_accessor<Self>;
 
 public:
-
   /// \name Constructors.
   //@{
 
@@ -215,34 +212,6 @@ public:
   }
 
   //@}
-
-protected:
-
-  /// \name Managing and notifying the arrangement observers.
-  //@{
-  typedef Arr_observer<Self>                      Observer;
-
-  /*!
-   * Register a new observer (so it starts receiving notifications).
-   * \param p_obs A pointer to the observer object.
-   */
-  void _register_observer (Observer *p_obs)
-  {
-    Base::_register_observer ((typename Base::Observer*)p_obs);
-    return;
-  }
-
-  /*!
-   * Unregister a new observer (so it stops receiving notifications).
-   * \param p_obs A pointer to the observer object.
-   * \return Whether the observer was successfully unregistered.
-   */
-  bool _unregister_observer (Observer *p_obs)
-  {
-    return (Base::_unregister_observer ((typename Base::Observer*)p_obs));
-  }
-  //@}
-
 };
 
 } //namespace CGAL
diff --git a/Arrangement_on_surface_2/include/CGAL/Arrangement_2/Arr_on_surface_with_history_2_impl.h b/Arrangement_on_surface_2/include/CGAL/Arrangement_2/Arr_on_surface_with_history_2_impl.h
index b60f198906f..8c63a3344c1 100644
--- a/Arrangement_on_surface_2/include/CGAL/Arrangement_2/Arr_on_surface_with_history_2_impl.h
+++ b/Arrangement_on_surface_2/include/CGAL/Arrangement_2/Arr_on_surface_with_history_2_impl.h
@@ -273,29 +273,6 @@ bool Arrangement_on_surface_with_history_2<GeomTr,TopTr>::are_mergeable
                                                         e2->curve()));
 }
 
-//-----------------------------------------------------------------------------
-// Register a new observer (so it starts receiving notifications).
-//
-template<class GeomTr, class TopTr>
-void Arrangement_on_surface_with_history_2<GeomTr,TopTr>::
-_register_observer(Arr_observer<Self> *p_obs)
-{
-  Base_arr_2::_register_observer
-    (reinterpret_cast<Arr_observer<Base_arr_2>*>(p_obs));
-  return;
-}
-
-//-----------------------------------------------------------------------------
-// Unregister an observer (so it stops receiving notifications).
-//
-template<class GeomTr, class TopTr>
-bool Arrangement_on_surface_with_history_2<GeomTr,TopTr>::
-_unregister_observer(Arr_observer<Self> *p_obs)
-{
-  return (Base_arr_2::_unregister_observer
-          (reinterpret_cast<Arr_observer<Base_arr_2>*>(p_obs)));
-}
-
 } //namespace CGAL
 
 #endif
diff --git a/Arrangement_on_surface_2/include/CGAL/Arrangement_on_surface_2.h b/Arrangement_on_surface_2/include/CGAL/Arrangement_on_surface_2.h
index ae87c8ddb28..f99be868ac0 100644
--- a/Arrangement_on_surface_2/include/CGAL/Arrangement_on_surface_2.h
+++ b/Arrangement_on_surface_2/include/CGAL/Arrangement_on_surface_2.h
@@ -109,6 +109,8 @@ public:
   typedef typename Topology_traits::Dcel            Dcel;
   typedef typename Dcel::Size                       Size;
 
+  typedef Arr_observer<Self>                        Observer;
+
 protected:
   friend class Arr_observer<Self>;
   friend class Arr_accessor<Self>;
@@ -892,7 +894,6 @@ protected:
   typedef CGAL_ALLOCATOR(Point_2)                 Points_alloc;
   typedef CGAL_ALLOCATOR(X_monotone_curve_2)      Curves_alloc;
 
-  typedef Arr_observer<Self>                      Observer;
   typedef std::list<Observer*>                    Observers_container;
   typedef typename Observers_container::iterator  Observers_iterator;
 
diff --git a/Arrangement_on_surface_2/include/CGAL/Arrangement_on_surface_with_history_2.h b/Arrangement_on_surface_2/include/CGAL/Arrangement_on_surface_with_history_2.h
index af560449961..ad1413f9f1c 100644
--- a/Arrangement_on_surface_2/include/CGAL/Arrangement_on_surface_with_history_2.h
+++ b/Arrangement_on_surface_2/include/CGAL/Arrangement_on_surface_with_history_2.h
@@ -26,7 +26,6 @@
 #include <CGAL/Arrangement_on_surface_2.h>
 #include <CGAL/Arr_overlay_2.h>
 #include <CGAL/Arr_consolidated_curve_data_traits_2.h>
-#include <CGAL/Arr_observer.h>
 #include <CGAL/In_place_list.h>
 #include <CGAL/Arrangement_2/Arr_with_history_accessor.h>
 
@@ -75,10 +74,7 @@ public:
   typedef typename Geometry_traits_2::Curve_2             Curve_2;
   typedef typename Geometry_traits_2::X_monotone_curve_2  X_monotone_curve_2;
 
-  typedef Arr_observer<Self>                              Observer;
-
 protected:
-  friend class Arr_observer<Self>;
   friend class Arr_accessor<Self>;
   friend class Arr_with_history_accessor<Self>;
 
@@ -582,24 +578,6 @@ public:
   //@}
 
 protected:
-
-  /// \name Managing and notifying the arrangement observers.
-  //@{
-
-  /*!
-   * Register a new observer (so it starts receiving notifications).
-   * \param p_obs A pointer to the observer object.
-   */
-  void _register_observer (Observer *p_obs);
-
-  /*!
-   * Unregister an observer (so it stops receiving notifications).
-   * \param p_obs A pointer to the observer object.
-   * \return Whether the observer was successfully unregistered.
-   */
-  bool _unregister_observer (Observer *p_obs);
-  //@}
-
   /// \name Curve insertion and deletion.
   //@{
 
diff --git a/Arrangement_on_surface_2/include/CGAL/Arrangement_with_history_2.h b/Arrangement_on_surface_2/include/CGAL/Arrangement_with_history_2.h
index 134c2f0c20e..24853e94b9c 100644
--- a/Arrangement_on_surface_2/include/CGAL/Arrangement_with_history_2.h
+++ b/Arrangement_on_surface_2/include/CGAL/Arrangement_with_history_2.h
@@ -119,7 +119,6 @@ public:
 private:
   typedef Arrangement_with_history_2<Geometry_traits_2, Dcel>   Self;
 
-  friend class Arr_observer<Self>;
   friend class Arr_accessor<Self>;
 
 public:
@@ -233,35 +232,8 @@ public:
 
     return (Face_const_handle (p_oc->face()));
   }
+
   //@}
-
-protected:
-
-  /// \name Managing and notifying the arrangement observers.
-  //@{
-  typedef Arr_observer<Self>                      Observer;
-
-  /*!
-   * Register a new observer (so it starts receiving notifications).
-   * \param p_obs A pointer to the observer object.
-   */
-  void _register_observer (Observer *p_obs)
-  {
-    Base::_register_observer ((typename Base::Observer*)p_obs);
-    return;
-  }
-
-  /*!
-   * Unregister a new observer (so it stops receiving notifications).
-   * \param p_obs A pointer to the observer object.
-   * \return Whether the observer was successfully unregistered.
-   */
-  bool _unregister_observer (Observer *p_obs)
-  {
-    return (Base::_unregister_observer ((typename Base::Observer*)p_obs));
-  }
-  //@}
-
 };
 
 } //namespace CGAL
diff --git a/Arrangement_on_surface_2/test/Arrangement_on_surface_2/test_observer.cpp b/Arrangement_on_surface_2/test/Arrangement_on_surface_2/test_observer.cpp
index 6f12f6e8c73..bc03ddc0957 100644
--- a/Arrangement_on_surface_2/test/Arrangement_on_surface_2/test_observer.cpp
+++ b/Arrangement_on_surface_2/test/Arrangement_on_surface_2/test_observer.cpp
@@ -5,7 +5,6 @@
 #include <CGAL/MP_Float.h>
 #include <CGAL/Arr_segment_traits_2.h>
 #include <CGAL/Arrangement_2.h>
-#include <CGAL/Arr_observer.h>
 #include <CGAL/Arr_enums.h>
 
 #include <iostream>
@@ -57,14 +56,11 @@ void compare_results(std::string str)
 
 // An arrangement observer, used to receive notifications of face splits and
 // face mergers.
-class Test_observer : public CGAL::Arr_observer<Arrangement_2>
-{
+class Test_observer : public Arrangement_2::Observer {
 
 public:
 
-  Test_observer (Arrangement_2& arr) :
-    CGAL::Arr_observer<Arrangement_2> (arr)
-  {}
+  Test_observer(Arrangement_2& arr) : Arrangement_2::Observer(arr) {}
 
   /// \name Notification functions on global arrangement operations.
   //@{
diff --git a/Envelope_3/include/CGAL/Envelope_3/Envelope_divide_and_conquer_3.h b/Envelope_3/include/CGAL/Envelope_3/Envelope_divide_and_conquer_3.h
index 9b3136be2b4..757ef127412 100644
--- a/Envelope_3/include/CGAL/Envelope_3/Envelope_divide_and_conquer_3.h
+++ b/Envelope_3/include/CGAL/Envelope_3/Envelope_divide_and_conquer_3.h
@@ -28,7 +28,6 @@
 #include <time.h>
 
 #include <CGAL/enum.h>
-#include <CGAL/Arr_observer.h>
 #include <CGAL/Envelope_3/Envelope_base.h>
 #include <CGAL/Envelope_3/Envelope_overlay_2.h>
 #include <CGAL/Envelope_3/Envelope_element_visitor_3.h>
@@ -134,7 +133,8 @@ protected:
   typedef typename Minimization_diagram_2::Inner_ccb_iterator
     Inner_ccb_iterator;
 
-  typedef Arr_observer<Minimization_diagram_2>          Md_observer;
+  typedef typename Minimization_diagram_2::Observer
+    Md_observer;
   typedef typename Minimization_diagram_2::Dcel::Dcel_data_iterator
     Envelope_data_iterator;
 
diff --git a/Envelope_3/include/CGAL/Envelope_3/Envelope_element_visitor_3.h b/Envelope_3/include/CGAL/Envelope_3/Envelope_element_visitor_3.h
index e5184ba9a2a..d5e118bf8d7 100644
--- a/Envelope_3/include/CGAL/Envelope_3/Envelope_element_visitor_3.h
+++ b/Envelope_3/include/CGAL/Envelope_3/Envelope_element_visitor_3.h
@@ -96,7 +96,7 @@ protected:
   typedef typename Minimization_diagram_2::Dcel::Dcel_data_iterator
                                                         Envelope_data_iterator;
 
-  typedef Arr_observer<Minimization_diagram_2>          Md_observer;
+  typedef typename Minimization_diagram_2::Observer     Md_observer;
   typedef Arr_accessor<Minimization_diagram_2>          Md_accessor;
 
   typedef typename Topology_traits::Default_point_location_strategy
diff --git a/Minkowski_sum_2/include/CGAL/Polygon_vertical_decomposition_2.h b/Minkowski_sum_2/include/CGAL/Polygon_vertical_decomposition_2.h
index 5db68c8abaa..d040ccb7158 100644
--- a/Minkowski_sum_2/include/CGAL/Polygon_vertical_decomposition_2.h
+++ b/Minkowski_sum_2/include/CGAL/Polygon_vertical_decomposition_2.h
@@ -77,9 +77,9 @@ private:
 
   // An arrangement observer, used to receive notifications of face splits and
   // face mergers.
-  class My_observer : public CGAL::Arr_observer<Arrangement_2> {
+  class My_observer : public Arrangement_2::Observer {
   public:
-    My_observer(Arrangement_2& arr) : Arr_observer<Arrangement_2>(arr) {}
+    My_observer(Arrangement_2& arr) : Arrangement_2::Observer(arr) {}
 
     virtual void after_split_face(Face_handle f, Face_handle new_f,
                                   bool /* is_hole */)
diff --git a/Visibility_2/include/CGAL/Triangular_expansion_visibility_2.h b/Visibility_2/include/CGAL/Triangular_expansion_visibility_2.h
index 408bf95c847..72465c5c97f 100644
--- a/Visibility_2/include/CGAL/Triangular_expansion_visibility_2.h
+++ b/Visibility_2/include/CGAL/Triangular_expansion_visibility_2.h
@@ -21,7 +21,6 @@
 #include <memory>
 #include <CGAL/boost/iterator/transform_iterator.hpp>
 #include <CGAL/Constrained_Delaunay_triangulation_2.h>
-#include <CGAL/Arr_observer.h>
 #include <CGAL/assertions.h>
 #include <CGAL/use.h>
 
@@ -86,10 +85,10 @@ private:
   };
 
   // Observer to track any changes of the attached arrangement.
-  class Observer : public Arr_observer<Arrangement_2>
+  class Observer : public Arrangement_2::Observer
   {
 
-      typedef Arr_observer<Arrangement_2>                           Base;
+      typedef typename Arrangement_2::Observer                      Base;
       typedef Observer                                              Self;