diff --git a/Arrangement_2/include/CGAL/Arr_point_location/Td_predicates.h b/Arrangement_2/include/CGAL/Arr_point_location/Td_predicates.h
index e31ec3478b3..01f9b961f3b 100644
--- a/Arrangement_2/include/CGAL/Arr_point_location/Td_predicates.h
+++ b/Arrangement_2/include/CGAL/Arr_point_location/Td_predicates.h
@@ -20,9 +20,7 @@
 #ifndef CGAL_TD_PREDICATES_H
 #define CGAL_TD_PREDICATES_H
 
-#ifndef CGAL_BASIC_H
 #include <CGAL/basic.h>
-#endif
 
 #include <functional>
 
diff --git a/Arrangement_2/include/CGAL/Arr_point_location/Trapezoidal_decomposition_2_misc.h b/Arrangement_2/include/CGAL/Arr_point_location/Trapezoidal_decomposition_2_misc.h
index f162c53f6f4..fcb747d7c46 100644
--- a/Arrangement_2/include/CGAL/Arr_point_location/Trapezoidal_decomposition_2_misc.h
+++ b/Arrangement_2/include/CGAL/Arr_point_location/Trapezoidal_decomposition_2_misc.h
@@ -24,9 +24,7 @@
 #include <CGAL/Handle.h>
 #include <CGAL/Arr_point_location/Td_ninetuple.h>
 #endif
-#ifndef CGAL_TWOTUPLE_H
 #include <CGAL/Twotuple.h>
-#endif
 
 #ifndef CGAL_TD_DAG_H
 #include <CGAL/Arr_point_location/Td_dag.h>
diff --git a/Circulator/include/CGAL/circulator_impl.h b/Circulator/include/CGAL/circulator_impl.h
index 906d8a1e5bc..41e7eb7c9e1 100644
--- a/Circulator/include/CGAL/circulator_impl.h
+++ b/Circulator/include/CGAL/circulator_impl.h
@@ -24,9 +24,7 @@
 #ifndef CGAL_CIRCULATOR_IMPL_H
 #define CGAL_CIRCULATOR_IMPL_H 1
 
-#ifndef CGAL_CIRCULATOR_H
 #include <CGAL/circulator.h>
-#endif
 
 CGAL_BEGIN_NAMESPACE
 
diff --git a/Conic_2/include/CGAL/Conic_2.h b/Conic_2/include/CGAL/Conic_2.h
index 109ec13fcfb..80d8740aefc 100644
--- a/Conic_2/include/CGAL/Conic_2.h
+++ b/Conic_2/include/CGAL/Conic_2.h
@@ -25,9 +25,7 @@
 #ifndef CGAL_CONIC_2_H
 #define CGAL_CONIC_2_H
 
-#ifndef CGAL_IO_FORWARD_DECL_WINDOW_STREAM_H
 #include <CGAL/IO/forward_decl_window_stream.h>
-#endif
 
 #include <CGAL/Conic_misc.h>
 
diff --git a/HalfedgeDS/include/CGAL/HalfedgeDS_face_base.h b/HalfedgeDS/include/CGAL/HalfedgeDS_face_base.h
index 8681030d440..1883bdc2e45 100644
--- a/HalfedgeDS/include/CGAL/HalfedgeDS_face_base.h
+++ b/HalfedgeDS/include/CGAL/HalfedgeDS_face_base.h
@@ -24,9 +24,7 @@
 #ifndef CGAL_HALFEDGEDS_FACE_BASE_H
 #define CGAL_HALFEDGEDS_FACE_BASE_H 1
 
-#ifndef CGAL_BASIC_H
 #include <CGAL/basic.h>
-#endif
 
 CGAL_BEGIN_NAMESPACE
 
diff --git a/HalfedgeDS/include/CGAL/HalfedgeDS_face_min_base.h b/HalfedgeDS/include/CGAL/HalfedgeDS_face_min_base.h
index 83c41419c93..852444f3693 100644
--- a/HalfedgeDS/include/CGAL/HalfedgeDS_face_min_base.h
+++ b/HalfedgeDS/include/CGAL/HalfedgeDS_face_min_base.h
@@ -24,9 +24,7 @@
 #ifndef CGAL_HALFEDGEDS_FACE_MIN_BASE_H
 #define CGAL_HALFEDGEDS_FACE_MIN_BASE_H 1
 
-#ifndef CGAL_BASIC_H
 #include <CGAL/basic.h>
-#endif
 
 CGAL_BEGIN_NAMESPACE
 
diff --git a/HalfedgeDS/include/CGAL/HalfedgeDS_halfedge_base.h b/HalfedgeDS/include/CGAL/HalfedgeDS_halfedge_base.h
index 27116363001..28667ed767d 100644
--- a/HalfedgeDS/include/CGAL/HalfedgeDS_halfedge_base.h
+++ b/HalfedgeDS/include/CGAL/HalfedgeDS_halfedge_base.h
@@ -24,9 +24,7 @@
 #ifndef CGAL_HALFEDGEDS_HALFEDGE_BASE_H
 #define CGAL_HALFEDGEDS_HALFEDGE_BASE_H 1
 
-#ifndef CGAL_BASIC_H
 #include <CGAL/basic.h>
-#endif
 
 CGAL_BEGIN_NAMESPACE
 
diff --git a/HalfedgeDS/include/CGAL/HalfedgeDS_halfedge_min_base.h b/HalfedgeDS/include/CGAL/HalfedgeDS_halfedge_min_base.h
index b9f830aeb31..9d670cc0f5b 100644
--- a/HalfedgeDS/include/CGAL/HalfedgeDS_halfedge_min_base.h
+++ b/HalfedgeDS/include/CGAL/HalfedgeDS_halfedge_min_base.h
@@ -24,9 +24,7 @@
 #ifndef CGAL_HALFEDGEDS_HALFEDGE_MIN_BASE_H
 #define CGAL_HALFEDGEDS_HALFEDGE_MIN_BASE_H 1
 
-#ifndef CGAL_BASIC_H
 #include <CGAL/basic.h>
-#endif
 
 CGAL_BEGIN_NAMESPACE
 
diff --git a/HalfedgeDS/include/CGAL/HalfedgeDS_items_2.h b/HalfedgeDS/include/CGAL/HalfedgeDS_items_2.h
index 17bf12bc0df..765527d40ef 100644
--- a/HalfedgeDS/include/CGAL/HalfedgeDS_items_2.h
+++ b/HalfedgeDS/include/CGAL/HalfedgeDS_items_2.h
@@ -23,15 +23,9 @@
 
 #ifndef CGAL_HALFEDGEDS_ITEMS_2_H
 #define CGAL_HALFEDGEDS_ITEMS_2_H 1
-#ifndef CGAL_HALFEDGEDS_VERTEX_BASE_H
 #include <CGAL/HalfedgeDS_vertex_base.h>
-#endif
-#ifndef CGAL_HALFEDGEDS_HALFEDGE_BASE_H
 #include <CGAL/HalfedgeDS_halfedge_base.h>
-#endif
-#ifndef CGAL_HALFEDGEDS_FACE_BASE_H
 #include <CGAL/HalfedgeDS_face_base.h>
-#endif
 
 CGAL_BEGIN_NAMESPACE
 
diff --git a/HalfedgeDS/include/CGAL/HalfedgeDS_iterator.h b/HalfedgeDS/include/CGAL/HalfedgeDS_iterator.h
index 804060f2450..93219f54567 100644
--- a/HalfedgeDS/include/CGAL/HalfedgeDS_iterator.h
+++ b/HalfedgeDS/include/CGAL/HalfedgeDS_iterator.h
@@ -23,9 +23,7 @@
 
 #ifndef CGAL_HALFEDGEDS_ITERATOR_H
 #define CGAL_HALFEDGEDS_ITERATOR_H 1
-#ifndef CGAL_CIRCULATOR_H
 #include <CGAL/circulator.h>
-#endif
 
 CGAL_BEGIN_NAMESPACE
 
diff --git a/HalfedgeDS/include/CGAL/HalfedgeDS_min_items.h b/HalfedgeDS/include/CGAL/HalfedgeDS_min_items.h
index e8ab9c16569..8b80a7eec36 100644
--- a/HalfedgeDS/include/CGAL/HalfedgeDS_min_items.h
+++ b/HalfedgeDS/include/CGAL/HalfedgeDS_min_items.h
@@ -23,15 +23,9 @@
 
 #ifndef CGAL_HALFEDGEDS_MIN_ITEMS_H
 #define CGAL_HALFEDGEDS_MIN_ITEMS_H 1
-#ifndef CGAL_HALFEDGEDS_VERTEX_MIN_BASE_H
 #include <CGAL/HalfedgeDS_vertex_min_base.h>
-#endif
-#ifndef CGAL_HALFEDGEDS_HALFEDGE_MIN_BASE_H
 #include <CGAL/HalfedgeDS_halfedge_min_base.h>
-#endif
-#ifndef CGAL_HALFEDGEDS_FACE_MIN_BASE_H
 #include <CGAL/HalfedgeDS_face_min_base.h>
-#endif
 
 CGAL_BEGIN_NAMESPACE
 
diff --git a/HalfedgeDS/include/CGAL/HalfedgeDS_vertex_base.h b/HalfedgeDS/include/CGAL/HalfedgeDS_vertex_base.h
index f1d2138479b..cda96f7640b 100644
--- a/HalfedgeDS/include/CGAL/HalfedgeDS_vertex_base.h
+++ b/HalfedgeDS/include/CGAL/HalfedgeDS_vertex_base.h
@@ -24,9 +24,7 @@
 #ifndef CGAL_HALFEDGEDS_VERTEX_BASE_H
 #define CGAL_HALFEDGEDS_VERTEX_BASE_H 1
 
-#ifndef CGAL_BASIC_H
 #include <CGAL/basic.h>
-#endif
 
 CGAL_BEGIN_NAMESPACE
 
diff --git a/HalfedgeDS/include/CGAL/HalfedgeDS_vertex_min_base.h b/HalfedgeDS/include/CGAL/HalfedgeDS_vertex_min_base.h
index e9279588f8d..8e7bad54022 100644
--- a/HalfedgeDS/include/CGAL/HalfedgeDS_vertex_min_base.h
+++ b/HalfedgeDS/include/CGAL/HalfedgeDS_vertex_min_base.h
@@ -24,9 +24,7 @@
 #ifndef CGAL_HALFEDGEDS_VERTEX_MIN_BASE_H
 #define CGAL_HALFEDGEDS_VERTEX_MIN_BASE_H 1
 
-#ifndef CGAL_BASIC_H
 #include <CGAL/basic.h>
-#endif
 
 CGAL_BEGIN_NAMESPACE
 
diff --git a/HalfedgeDS/include/CGAL/Polyhedron_items_3.h b/HalfedgeDS/include/CGAL/Polyhedron_items_3.h
index ca77ad248da..71526e01917 100644
--- a/HalfedgeDS/include/CGAL/Polyhedron_items_3.h
+++ b/HalfedgeDS/include/CGAL/Polyhedron_items_3.h
@@ -23,15 +23,9 @@
 
 #ifndef CGAL_POLYHEDRON_ITEMS_3_H
 #define CGAL_POLYHEDRON_ITEMS_3_H 1
-#ifndef CGAL_HALFEDGEDS_VERTEX_BASE_H
 #include <CGAL/HalfedgeDS_vertex_base.h>
-#endif
-#ifndef CGAL_HALFEDGEDS_HALFEDGE_BASE_H
 #include <CGAL/HalfedgeDS_halfedge_base.h>
-#endif
-#ifndef CGAL_HALFEDGEDS_FACE_BASE_H
 #include <CGAL/HalfedgeDS_face_base.h>
-#endif
 
 CGAL_BEGIN_NAMESPACE
 
diff --git a/HalfedgeDS/test/HalfedgeDS/test_hds.C b/HalfedgeDS/test/HalfedgeDS/test_hds.C
index 106d0e885dd..3131796c522 100644
--- a/HalfedgeDS/test/HalfedgeDS/test_hds.C
+++ b/HalfedgeDS/test/HalfedgeDS/test_hds.C
@@ -103,9 +103,7 @@ void test_HalfedgeDS_default() {
     CGAL_assertion(!(i == hds.halfedges_end()));
 }
 
-#ifndef CGAL_HALFEDGEDS_VECTOR_H
 #include <CGAL/HalfedgeDS_vector.h>
-#endif
 
 void test_HalfedgeDS_vector() {
     // Instantiation of the halfedge data structure using vector
diff --git a/Min_annulus_d/include/CGAL/Min_annulus_d.h b/Min_annulus_d/include/CGAL/Min_annulus_d.h
index 152eeba9970..0fd213f7aaa 100644
--- a/Min_annulus_d/include/CGAL/Min_annulus_d.h
+++ b/Min_annulus_d/include/CGAL/Min_annulus_d.h
@@ -22,13 +22,9 @@
 
 // includes
 // --------
-#ifndef CGAL_OPTIMISATION_BASIC_H
 #  include <CGAL/Optimisation/basic.h>
-#endif
 
-#ifndef CGAL_FUNCTION_OBJECTS_H
 #  include <CGAL/function_objects.h>
-#endif
 
 #include <CGAL/QP_solver.h>
 #include <CGAL/QP_solver/QP_partial_filtered_pricing.h>
diff --git a/Min_annulus_d/test/Min_annulus_d/test_Min_annulus_d.h b/Min_annulus_d/test/Min_annulus_d/test_Min_annulus_d.h
index b5b5fe04945..d87bd6d8fc0 100644
--- a/Min_annulus_d/test/Min_annulus_d/test_Min_annulus_d.h
+++ b/Min_annulus_d/test/Min_annulus_d/test_Min_annulus_d.h
@@ -29,9 +29,7 @@
 #define CGAL_TEST_MIN_ANNULUS_D_H
 
 // includes
-#ifndef CGAL_IO_VERBOSE_OSTREAM_H
 #  include <CGAL/IO/Verbose_ostream.h>
-#endif
 #include <cassert>
 
 CGAL_BEGIN_NAMESPACE
diff --git a/Min_circle_2/include/CGAL/Min_circle_2/Min_circle_2_adapterC2.h b/Min_circle_2/include/CGAL/Min_circle_2/Min_circle_2_adapterC2.h
index a698b115776..cbd65f26655 100644
--- a/Min_circle_2/include/CGAL/Min_circle_2/Min_circle_2_adapterC2.h
+++ b/Min_circle_2/include/CGAL/Min_circle_2/Min_circle_2_adapterC2.h
@@ -21,9 +21,7 @@
 #define CGAL_MIN_CIRCLE_2_ADAPTERC2_H
 
 // includes
-#ifndef CGAL_OPTIMISATION_BASIC_H
 #  include <CGAL/Optimisation/basic.h>
-#endif
 
 CGAL_BEGIN_NAMESPACE
 
diff --git a/Min_circle_2/include/CGAL/Min_circle_2/Min_circle_2_adapterH2.h b/Min_circle_2/include/CGAL/Min_circle_2/Min_circle_2_adapterH2.h
index 0ffe660da92..4f2d59912bf 100644
--- a/Min_circle_2/include/CGAL/Min_circle_2/Min_circle_2_adapterH2.h
+++ b/Min_circle_2/include/CGAL/Min_circle_2/Min_circle_2_adapterH2.h
@@ -21,9 +21,7 @@
 #define CGAL_MIN_CIRCLE_2_ADAPTERH2_H
 
 // includes
-#ifndef CGAL_OPTIMISATION_BASIC_H
 #  include <CGAL/Optimisation/basic.h>
-#endif
 
 CGAL_BEGIN_NAMESPACE
 
diff --git a/Min_circle_2/include/CGAL/Min_circle_2/Optimisation_circle_2.h b/Min_circle_2/include/CGAL/Min_circle_2/Optimisation_circle_2.h
index b44c8d116de..9487827f654 100644
--- a/Min_circle_2/include/CGAL/Min_circle_2/Optimisation_circle_2.h
+++ b/Min_circle_2/include/CGAL/Min_circle_2/Optimisation_circle_2.h
@@ -22,12 +22,8 @@
 
 // includes
 
-#ifndef CGAL_BASIC_CONSTRUCTIONS_2_H
 #  include <CGAL/basic_constructions_2.h>
-#endif
-#ifndef CGAL_SQUARED_DISTANCE_2_H
 #  include <CGAL/squared_distance_2.h>
-#endif
 
 CGAL_BEGIN_NAMESPACE
 
diff --git a/Min_circle_2/include/CGAL/Min_circle_2/Optimisation_circle_2_impl.h b/Min_circle_2/include/CGAL/Min_circle_2/Optimisation_circle_2_impl.h
index cc286514e25..a9c1d05d7b1 100644
--- a/Min_circle_2/include/CGAL/Min_circle_2/Optimisation_circle_2_impl.h
+++ b/Min_circle_2/include/CGAL/Min_circle_2/Optimisation_circle_2_impl.h
@@ -18,9 +18,7 @@
 // Author(s)     : Sven Schoenherr <sven@inf.ethz.ch>, Bernd Gaertner
 
 // includes
-#ifndef CGAL_OPTIMISATION_ASSERTIONS_H
 #  include <CGAL/Optimisation/assertions.h>
-#endif
 
 CGAL_BEGIN_NAMESPACE
 
diff --git a/Min_circle_2/include/CGAL/Min_circle_2_traits_2.h b/Min_circle_2/include/CGAL/Min_circle_2_traits_2.h
index 650fe982dee..79d7def2ba8 100644
--- a/Min_circle_2/include/CGAL/Min_circle_2_traits_2.h
+++ b/Min_circle_2/include/CGAL/Min_circle_2_traits_2.h
@@ -25,9 +25,7 @@
 #ifndef CGAL_OPTIMISATION_CIRCLE_2_H
 #  include <CGAL/Min_circle_2/Optimisation_circle_2.h>
 #endif
-#ifndef CGAL_PREDICATES_ON_POINTS_2_H
 #  include <CGAL/predicates_on_points_2.h>
-#endif
 
 CGAL_BEGIN_NAMESPACE
 
diff --git a/Min_ellipse_2/include/CGAL/Min_ellipse_2/Min_ellipse_2_adapterC2.h b/Min_ellipse_2/include/CGAL/Min_ellipse_2/Min_ellipse_2_adapterC2.h
index 9564c94e1ba..cf47003a3b9 100644
--- a/Min_ellipse_2/include/CGAL/Min_ellipse_2/Min_ellipse_2_adapterC2.h
+++ b/Min_ellipse_2/include/CGAL/Min_ellipse_2/Min_ellipse_2_adapterC2.h
@@ -21,12 +21,8 @@
 #define CGAL_MIN_ELLIPSE_2_ADAPTERC2_H
 
 // includes
-#ifndef CGAL_CONICCPA2_H
 #  include <CGAL/ConicCPA2.h>
-#endif
-#ifndef CGAL_OPTIMISATION_ASSERTIONS_H
 #  include <CGAL/Optimisation/assertions.h>
-#endif
 
 CGAL_BEGIN_NAMESPACE
 
diff --git a/Min_ellipse_2/include/CGAL/Min_ellipse_2/Min_ellipse_2_adapterH2.h b/Min_ellipse_2/include/CGAL/Min_ellipse_2/Min_ellipse_2_adapterH2.h
index db92157b99f..664a058a124 100644
--- a/Min_ellipse_2/include/CGAL/Min_ellipse_2/Min_ellipse_2_adapterH2.h
+++ b/Min_ellipse_2/include/CGAL/Min_ellipse_2/Min_ellipse_2_adapterH2.h
@@ -21,12 +21,8 @@
 #define CGAL_MIN_ELLIPSE_2_ADAPTERH2_H
 
 // includes
-#ifndef CGAL_CONICHPA2_H
 #  include <CGAL/ConicHPA2.h>
-#endif
-#ifndef CGAL_OPTIMISATION_ASSERTIONS_H
 #  include <CGAL/Optimisation/assertions.h>
-#endif
 
 CGAL_BEGIN_NAMESPACE
 
diff --git a/Min_ellipse_2/include/CGAL/Min_ellipse_2/Optimisation_ellipse_2.h b/Min_ellipse_2/include/CGAL/Min_ellipse_2/Optimisation_ellipse_2.h
index 521156596c5..8999a32bddb 100644
--- a/Min_ellipse_2/include/CGAL/Min_ellipse_2/Optimisation_ellipse_2.h
+++ b/Min_ellipse_2/include/CGAL/Min_ellipse_2/Optimisation_ellipse_2.h
@@ -23,9 +23,7 @@
 #include <CGAL/Conic_2.h>
 #include <CGAL/Optimisation/assertions.h>
 
-#ifndef CGAL_IO_FORWARD_DECL_WINDOW_STREAM_H
 #  include <CGAL/IO/forward_decl_window_stream.h>
-#endif
 
 CGAL_BEGIN_NAMESPACE
 
diff --git a/Min_sphere_d_new/include/CGAL/Min_sphere_d_new.h b/Min_sphere_d_new/include/CGAL/Min_sphere_d_new.h
index eaa2ce1cf5e..ceca79e021e 100644
--- a/Min_sphere_d_new/include/CGAL/Min_sphere_d_new.h
+++ b/Min_sphere_d_new/include/CGAL/Min_sphere_d_new.h
@@ -22,9 +22,7 @@
 
 // includes
 // --------
-#ifndef CGAL_OPTIMISATION_BASIC_H
 #  include <CGAL/Optimisation/basic.h>
-#endif
 
 #include <CGAL/function_objects.h>
 #include <CGAL/QP_solver.h>
diff --git a/Min_sphere_d_new/test/Min_sphere_d_new/test_Min_sphere_d.h b/Min_sphere_d_new/test/Min_sphere_d_new/test_Min_sphere_d.h
index a0b22154ff1..8673a35e1ce 100644
--- a/Min_sphere_d_new/test/Min_sphere_d_new/test_Min_sphere_d.h
+++ b/Min_sphere_d_new/test/Min_sphere_d_new/test_Min_sphere_d.h
@@ -29,9 +29,7 @@
 #define CGAL_TEST_MIN_SPHERE_D_H
 
 // includes
-#ifndef CGAL_IO_VERBOSE_OSTREAM_H
 #  include <CGAL/IO/Verbose_ostream.h>
-#endif
 #include <cassert>
 
 CGAL_BEGIN_NAMESPACE
diff --git a/Modifier/test/Modifier/test_modifier.C b/Modifier/test/Modifier/test_modifier.C
index 9b2db449732..bb346689bb5 100644
--- a/Modifier/test/Modifier/test_modifier.C
+++ b/Modifier/test/Modifier/test_modifier.C
@@ -25,12 +25,8 @@
 // ============================================================================
 
 
-#ifndef CGAL_BASIC_H
 #include <CGAL/basic.h>
-#endif // CGAL_BASIC_H
-#ifndef CGAL_MODIFIER_BASE_H
 #include <CGAL/Modifier_base.h>
-#endif // CGAL_MODIFIER_BASE_H
 
 using CGAL::Modifier_base;
 
diff --git a/Optimisation_basic/include/CGAL/Optimisation_d_traits_2.h b/Optimisation_basic/include/CGAL/Optimisation_d_traits_2.h
index 405ba946202..f0cafcd600f 100644
--- a/Optimisation_basic/include/CGAL/Optimisation_d_traits_2.h
+++ b/Optimisation_basic/include/CGAL/Optimisation_d_traits_2.h
@@ -25,15 +25,9 @@
 #define CGAL_OPTIMISATION_D_TRAITS_2_H
 
 // includes
-#ifndef CGAL_OPTIMISATION_ACCESS_DIMENSION_2_H
 #  include <CGAL/Optimisation/Access_dimension_2.h>
-#endif
-#ifndef CGAL_OPTIMISATION_ACCESS_COORDINATES_BEGIN_2_H
 #  include <CGAL/Optimisation/Access_coordinates_begin_2.h>
-#endif
-#ifndef CGAL_OPTIMISATION_CONSTRUCT_POINT_2_H
 #  include <CGAL/Optimisation/Construct_point_2.h>
-#endif
 
 CGAL_BEGIN_NAMESPACE
 
diff --git a/Optimisation_basic/include/CGAL/Optimisation_d_traits_3.h b/Optimisation_basic/include/CGAL/Optimisation_d_traits_3.h
index bb22507bf60..0d9a134eb73 100644
--- a/Optimisation_basic/include/CGAL/Optimisation_d_traits_3.h
+++ b/Optimisation_basic/include/CGAL/Optimisation_d_traits_3.h
@@ -25,15 +25,9 @@
 #define CGAL_OPTIMISATION_D_TRAITS_3_H
 
 // includes
-#ifndef CGAL_OPTIMISATION_ACCESS_DIMENSION_3_H
 #  include <CGAL/Optimisation/Access_dimension_3.h>
-#endif
-#ifndef CGAL_OPTIMISATION_ACCESS_COORDINATES_BEGIN_3_H
 #  include <CGAL/Optimisation/Access_coordinates_begin_3.h>
-#endif
-#ifndef CGAL_OPTIMISATION_CONSTRUCT_POINT_3_H
 #  include <CGAL/Optimisation/Construct_point_3.h>
-#endif
 
 CGAL_BEGIN_NAMESPACE
 
diff --git a/Optimisation_basic/include/CGAL/Optimisation_d_traits_d.h b/Optimisation_basic/include/CGAL/Optimisation_d_traits_d.h
index e372d9748dc..87562663871 100644
--- a/Optimisation_basic/include/CGAL/Optimisation_d_traits_d.h
+++ b/Optimisation_basic/include/CGAL/Optimisation_d_traits_d.h
@@ -25,15 +25,9 @@
 #define CGAL_OPTIMISATION_D_TRAITS_D_H
 
 // includes
-#ifndef CGAL_OPTIMISATION_ACCESS_DIMENSION_D_H
 #  include <CGAL/Optimisation/Access_dimension_d.h>
-#endif
-#ifndef CGAL_OPTIMISATION_ACCESS_COORDINATES_BEGIN_D_H
 #  include <CGAL/Optimisation/Access_coordinates_begin_d.h>
-#endif
-#ifndef CGAL_OPTIMISATION_CONSTRUCT_POINT_D_H
 #  include <CGAL/Optimisation/Construct_point_d.h>
-#endif
 
 CGAL_BEGIN_NAMESPACE
 
diff --git a/Polyhedron/include/CGAL/Polyhedron_min_items_3.h b/Polyhedron/include/CGAL/Polyhedron_min_items_3.h
index 40863dd9867..d0d8466a851 100644
--- a/Polyhedron/include/CGAL/Polyhedron_min_items_3.h
+++ b/Polyhedron/include/CGAL/Polyhedron_min_items_3.h
@@ -19,15 +19,9 @@
 
 #ifndef CGAL_POLYHEDRON_MIN_ITEMS_3_H
 #define CGAL_POLYHEDRON_MIN_ITEMS_3_H 1
-#ifndef CGAL_HALFEDGEDS_VERTEX_BASE_H
 #include <CGAL/HalfedgeDS_vertex_base.h>
-#endif
-#ifndef CGAL_HALFEDGEDS_HALFEDGE_BASE_H
 #include <CGAL/HalfedgeDS_halfedge_base.h>
-#endif
-#ifndef CGAL_HALFEDGEDS_FACE_BASE_H
 #include <CGAL/HalfedgeDS_face_base.h>
-#endif
 
 CGAL_BEGIN_NAMESPACE
 
diff --git a/Polytope_distance_d/test/Polytope_distance_d/test_Polytope_distance_d.h b/Polytope_distance_d/test/Polytope_distance_d/test_Polytope_distance_d.h
index 2c0cd82ee94..14a7f323689 100644
--- a/Polytope_distance_d/test/Polytope_distance_d/test_Polytope_distance_d.h
+++ b/Polytope_distance_d/test/Polytope_distance_d/test_Polytope_distance_d.h
@@ -28,9 +28,7 @@
 #define CGAL_TEST_POLYTOPE_DISTANCE_D_H
 
 // includes
-#ifndef CGAL_IO_VERBOSE_OSTREAM_H
 #  include <CGAL/IO/Verbose_ostream.h>
-#endif
 #include <cassert>
 
 CGAL_BEGIN_NAMESPACE
diff --git a/SearchStructures/include/CGAL/Tree_assertions.h b/SearchStructures/include/CGAL/Tree_assertions.h
index 0da77367398..eef850ceeff 100644
--- a/SearchStructures/include/CGAL/Tree_assertions.h
+++ b/SearchStructures/include/CGAL/Tree_assertions.h
@@ -21,9 +21,7 @@
 #ifndef CGAL_TREE_ASSERTIONS_H
 #define CGAL_TREE_ASSERTIONS_H
 
-#ifndef CGAL_ASSERTIONS_H
 #  include <CGAL/assertions.h>
-#endif
 
 
 // macro definitions
diff --git a/Straight_skeleton_2/include/CGAL/Straight_skeleton_items_2.h b/Straight_skeleton_2/include/CGAL/Straight_skeleton_items_2.h
index ce0e44c11e7..33b019e4607 100644
--- a/Straight_skeleton_2/include/CGAL/Straight_skeleton_items_2.h
+++ b/Straight_skeleton_2/include/CGAL/Straight_skeleton_items_2.h
@@ -18,17 +18,11 @@
 #ifndef CGAL_STRAIGHT_SKELETON_ITEMS_2_H
 #define CGAL_STRAIGHT_SKELETON_ITEMS_2_H 1
 
-#ifndef CGAL_STRAIGHT_SKELETON_VERTEX_BASE_2_H
 #include <CGAL/Straight_skeleton_vertex_base_2.h>
-#endif
 
-#ifndef CGAL_STRAIGHT_SKELETON_HALFEDGE_BASE_2_H
 #include <CGAL/Straight_skeleton_halfedge_base_2.h>
-#endif
 
-#ifndef CGAL_HALFEDGEDS_FACE_BASE_H
 #include <CGAL/HalfedgeDS_face_base.h>
-#endif
 
 CGAL_BEGIN_NAMESPACE
 
diff --git a/Straight_skeleton_2/include/CGAL/Straight_skeleton_vertex_base_2.h b/Straight_skeleton_2/include/CGAL/Straight_skeleton_vertex_base_2.h
index 8d4f2f34856..7bcc87d4439 100644
--- a/Straight_skeleton_2/include/CGAL/Straight_skeleton_vertex_base_2.h
+++ b/Straight_skeleton_2/include/CGAL/Straight_skeleton_vertex_base_2.h
@@ -18,13 +18,9 @@
 #ifndef CGAL_STRAIGHT_SKELETON_VERTEX_BASE_2_H
 #define CGAL_STRAIGHT_SKELETON_VERTEX_BASE_2_H 1
 
-#ifndef CGAL_STRAIGHT_SKELETON_HALFEDGE_BASE_2_H
 #include <CGAL/Straight_skeleton_halfedge_base_2.h>
-#endif
 
-#ifndef CGAL_CIRCULATOR_H
 #include <CGAL/circulator.h>
-#endif
 
 #include <boost/iterator/iterator_facade.hpp>
 
diff --git a/Stream_support/include/CGAL/IO/File_header_extended_OFF.h b/Stream_support/include/CGAL/IO/File_header_extended_OFF.h
index 71fac7048b7..63b4e21f877 100644
--- a/Stream_support/include/CGAL/IO/File_header_extended_OFF.h
+++ b/Stream_support/include/CGAL/IO/File_header_extended_OFF.h
@@ -24,9 +24,7 @@
 
 #ifndef CGAL_IO_FILE_HEADER_EXTENDED_OFF_H
 #define CGAL_IO_FILE_HEADER_EXTENDED_OFF_H 1
-#ifndef CGAL_BASIC_H
 #include <CGAL/basic.h>
-#endif
 
 #include <iostream>
 #include <string>
diff --git a/Stream_support/include/CGAL/IO/Writer_OFF.h b/Stream_support/include/CGAL/IO/Writer_OFF.h
index a1efd739562..e1a79cd917e 100644
--- a/Stream_support/include/CGAL/IO/Writer_OFF.h
+++ b/Stream_support/include/CGAL/IO/Writer_OFF.h
@@ -23,12 +23,8 @@
 
 #ifndef CGAL_IO_WRITER_OFF_H
 #define CGAL_IO_WRITER_OFF_H 1
-#ifndef CGAL_IO_FILE_WRITER_OFF_H
 #include <CGAL/IO/File_writer_OFF.h>
-#endif // CGAL_IO_FILE_WRITER_OFF_H
-#ifndef CGAL_IO_GENERIC_WRITER_H
 #include <CGAL/IO/Generic_writer.h>
-#endif // CGAL_IO_GENERIC_WRITER_H
 
 CGAL_BEGIN_NAMESPACE
 
diff --git a/Stream_support/src/CGAL/File_writer_inventor.cpp b/Stream_support/src/CGAL/File_writer_inventor.cpp
index 3db9c2e8fee..1f2002fa1fe 100644
--- a/Stream_support/src/CGAL/File_writer_inventor.cpp
+++ b/Stream_support/src/CGAL/File_writer_inventor.cpp
@@ -21,9 +21,7 @@
 //
 // Author(s)     : Lutz Kettner  <kettner@mpi-sb.mpg.de>
 
-#ifndef CGAL_IO_FILE_WRITER_INVENTOR_H
 #include <CGAL/IO/File_writer_inventor.h>
-#endif // CGAL_IO_FILE_WRITER_INVENTOR_H
 
 CGAL_BEGIN_NAMESPACE
 
diff --git a/Visibility_complex/include/CGAL/Visibility_complex_iterators.h b/Visibility_complex/include/CGAL/Visibility_complex_iterators.h
index dcd85a3e015..3dd00d4fd85 100644
--- a/Visibility_complex/include/CGAL/Visibility_complex_iterators.h
+++ b/Visibility_complex/include/CGAL/Visibility_complex_iterators.h
@@ -20,9 +20,7 @@
 #ifndef CGAL_VISIBILITY_COMPLEX_ITERATORS_H
 #define CGAL_VISIBILITY_COMPLEX_ITERATORS_H
 
-#ifndef CGAL_BASIC_H
 #include <CGAL/basic.h>
-#endif
 
 #include <list>
 
diff --git a/window/include/CGAL/IO/window_stream_xy_3.h b/window/include/CGAL/IO/window_stream_xy_3.h
index bd127cca36a..5937583d7ee 100644
--- a/window/include/CGAL/IO/window_stream_xy_3.h
+++ b/window/include/CGAL/IO/window_stream_xy_3.h
@@ -72,19 +72,13 @@
 #if defined(CGAL_LINE_3_H) || defined(CGAL_RAY_3_H) \
     || defined(CGAL_SEGMENT_3_H) || defined(CGAL_TRIANGLE_3_H) \
     || defined(CGAL_TETRAHEDRON_3_H)
-#ifndef CGAL_POINT_2_H
 #include <CGAL/Point_2.h>
-#endif
-#ifndef CGAL_POINT_3_H
 #include <CGAL/Point_3.h>
 #endif
-#endif
 
 // Define necessary 2d stream operators.
 // -------------------------------------
-#ifndef CGAL_IO_WINDOW_STREAM_H
 #include <CGAL/IO/Window_stream.h>
-#endif // CGAL_IO_WINDOW_STREAM_H
 
 CGAL_BEGIN_NAMESPACE