diff --git a/common/3d/clipper.h b/common/3d/clipper.h
index 2031a3659..7fe01c28a 100644
--- a/common/3d/clipper.h
+++ b/common/3d/clipper.h
@@ -19,10 +19,15 @@ COPYRIGHT 1993-1998 PARALLAX SOFTWARE CORPORATION.  ALL RIGHTS RESERVED.
 #ifndef _CLIPPER_H
 #define _CLIPPER_H
 
+#include "pstypes.h"
+
 #ifdef __cplusplus
 extern "C" {
 #endif
 
+struct g3s_codes;
+struct g3s_point;
+
 extern void free_temp_point(g3s_point *p);
 extern g3s_point **clip_polygon(g3s_point **src,g3s_point **dest,int *nv,g3s_codes *cc);
 extern void init_free_points(void);
diff --git a/common/3d/globvars.h b/common/3d/globvars.h
index 89e45a970..406c91579 100644
--- a/common/3d/globvars.h
+++ b/common/3d/globvars.h
@@ -21,6 +21,12 @@ COPYRIGHT 1993-1998 PARALLAX SOFTWARE CORPORATION.  ALL RIGHTS RESERVED.
 
 #define MAX_POINTS_IN_POLY 100
 
+#include "maths.h"
+
+struct vms_vector;
+struct vms_matrix;
+struct g3s_point;
+
 extern int Canvas_width,Canvas_height;	//the actual width & height
 extern fix Canv_w2,Canv_h2;				//fixed-point width,height/2
 
diff --git a/common/include/editor/esegment.h b/common/include/editor/esegment.h
index 8fd8440da..74a5bbdc1 100644
--- a/common/include/editor/esegment.h
+++ b/common/include/editor/esegment.h
@@ -1,5 +1,6 @@
 #pragma once
 #include "segment.h"
+#include "editor/editor.h"
 
 extern	segment  *Cursegp;				// Pointer to current segment in the mine, the one to which things happen.
 #ifdef DXX_BUILD_DESCENT_II
diff --git a/common/include/editor/meddraw.h b/common/include/editor/meddraw.h
index e4f2d2fb1..47fa20ab8 100644
--- a/common/include/editor/meddraw.h
+++ b/common/include/editor/meddraw.h
@@ -24,6 +24,10 @@ COPYRIGHT 1993-1998 PARALLAX SOFTWARE CORPORATION.  ALL RIGHTS RESERVED.
 extern "C" {
 #endif
 
+struct grs_canvas;
+struct editor_view;
+struct segment;
+
 void meddraw_init_views( grs_canvas * canvas);
 void draw_world(grs_canvas *screen_canvas,editor_view *v,segment *mine_ptr,int depth);
 void find_segments(short x,short y,grs_canvas *screen_canvas,editor_view *v,segment *mine_ptr,int depth);
diff --git a/common/include/editor/medmisc.h b/common/include/editor/medmisc.h
index 6009f2671..4fcbcf661 100644
--- a/common/include/editor/medmisc.h
+++ b/common/include/editor/medmisc.h
@@ -24,6 +24,9 @@ COPYRIGHT 1993-1998 PARALLAX SOFTWARE CORPORATION.  ALL RIGHTS RESERVED.
 extern "C" {
 #endif
 
+struct segment;
+struct vms_matrix;
+
 void GetMouseRotation( int idx, int idy, vms_matrix * RotMat );
 extern int Gameview_lockstep;                         //In medmisc.c
 int ToggleLockstep();
diff --git a/common/include/editor/medsel.h b/common/include/editor/medsel.h
index 5748ff5e6..d09b7c37b 100644
--- a/common/include/editor/medsel.h
+++ b/common/include/editor/medsel.h
@@ -24,6 +24,8 @@ COPYRIGHT 1993-1998 PARALLAX SOFTWARE CORPORATION.  ALL RIGHTS RESERVED.
 extern "C" {
 #endif
 
+struct vms_vector;
+
 extern void sort_seg_list(int n_segs,short *segnumlist,vms_vector *pos);
 
 #ifdef __cplusplus
diff --git a/common/include/editor/objpage.h b/common/include/editor/objpage.h
index 0041ae5cb..321563b9f 100644
--- a/common/include/editor/objpage.h
+++ b/common/include/editor/objpage.h
@@ -27,6 +27,7 @@ extern "C" {
 #endif
 
 struct d_event;
+struct vms_angvec;
 
 int objpage_grab_current(int n);
 int objpage_goto_first();
diff --git a/common/include/editor/seguvs.h b/common/include/editor/seguvs.h
index c70f54513..e6e6ce879 100644
--- a/common/include/editor/seguvs.h
+++ b/common/include/editor/seguvs.h
@@ -24,6 +24,8 @@ COPYRIGHT 1993-1998 PARALLAX SOFTWARE CORPORATION.  ALL RIGHTS RESERVED.
 extern "C" {
 #endif
 
+struct segment;
+
 extern void assign_light_to_side(segment *sp, int sidenum);
 extern void assign_default_lighting_all(void);
 extern void stretch_uvs_from_curedge(segment *segp, int side);
diff --git a/common/include/palette.h b/common/include/palette.h
index 218e605f9..9313d37c5 100644
--- a/common/include/palette.h
+++ b/common/include/palette.h
@@ -21,6 +21,8 @@ COPYRIGHT 1993-1998 PARALLAX SOFTWARE CORPORATION.  ALL RIGHTS RESERVED.
 #ifndef _PALETTE_H
 #define _PALETTE_H
 
+#include "pstypes.h"
+
 #ifdef __cplusplus
 extern "C" {
 #endif
diff --git a/common/include/pcx.h b/common/include/pcx.h
index 740d29b13..3e4c4dbfc 100644
--- a/common/include/pcx.h
+++ b/common/include/pcx.h
@@ -20,10 +20,14 @@ COPYRIGHT 1993-1998 PARALLAX SOFTWARE CORPORATION.  ALL RIGHTS RESERVED.
 #ifndef _PCX_H
 #define _PCX_H
 
+#include "pstypes.h"
+
 #ifdef __cplusplus
 extern "C" {
 #endif
 
+struct grs_bitmap;
+
 #define PCX_ERROR_NONE          0
 #define PCX_ERROR_OPENING       1
 #define PCX_ERROR_NO_HEADER     2
diff --git a/common/include/strio.h b/common/include/strio.h
index 7a0a24791..a3ee006cb 100644
--- a/common/include/strio.h
+++ b/common/include/strio.h
@@ -2,6 +2,8 @@
 #ifndef _STRIO_H
 #define _STRIO_H
 
+#include <physfs.h>
+
 #ifdef __cplusplus
 extern "C" {
 #endif
diff --git a/common/include/ui.h b/common/include/ui.h
index d6cf7aabe..6051d59f9 100644
--- a/common/include/ui.h
+++ b/common/include/ui.h
@@ -27,6 +27,9 @@ COPYRIGHT 1993-1999 PARALLAX SOFTWARE CORPORATION.  ALL RIGHTS RESERVED.
 extern "C" {
 #endif
 
+struct grs_bitmap;
+struct grs_canvas;
+struct grs_font;
 struct window;
 
 typedef struct {
diff --git a/common/main/automap.h b/common/main/automap.h
index 8a771ec7f..3921d4fcc 100644
--- a/common/main/automap.h
+++ b/common/main/automap.h
@@ -20,6 +20,9 @@ COPYRIGHT 1993-1999 PARALLAX SOFTWARE CORPORATION.  ALL RIGHTS RESERVED.
 #ifndef _AUTOMAP_H
 #define _AUTOMAP_H
 
+#include "pstypes.h"
+#include "segment.h"
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -32,7 +35,9 @@ extern void automap_clear_visited();
 extern ubyte Automap_visited[MAX_SEGMENTS];
 
 #if defined(DXX_BUILD_DESCENT_II)
-#include "object.h"
+struct object;
+struct vms_vector;
+
 void DropBuddyMarker(object *objp);
 void InitMarkerInput();
 int MarkerInputMessage(int key);
diff --git a/common/main/controls.h b/common/main/controls.h
index 64fc4b62d..10da711d9 100644
--- a/common/main/controls.h
+++ b/common/main/controls.h
@@ -21,10 +21,14 @@ COPYRIGHT 1993-1999 PARALLAX SOFTWARE CORPORATION.  ALL RIGHTS RESERVED.
 #ifndef _CONTROLS_H
 #define _CONTROLS_H
 
+#include "maths.h"
+
 #ifdef __cplusplus
 extern "C" {
 #endif
 
+struct object;
+
 void read_flying_controls( object * obj );
 
 #if defined(DXX_BUILD_DESCENT_II)
diff --git a/common/main/escort.h b/common/main/escort.h
index 3f3540617..0ca971252 100644
--- a/common/main/escort.h
+++ b/common/main/escort.h
@@ -7,6 +7,8 @@
 #ifndef _ESCORT_H
 #define _ESCORT_H
 
+#include "maths.h"
+
 #ifdef __cplusplus
 extern "C" {
 #endif
diff --git a/common/main/fireball.h b/common/main/fireball.h
index fac9e1785..4e9564280 100644
--- a/common/main/fireball.h
+++ b/common/main/fireball.h
@@ -21,10 +21,16 @@ COPYRIGHT 1993-1999 PARALLAX SOFTWARE CORPORATION.  ALL RIGHTS RESERVED.
 #ifndef _FIREBALL_H
 #define _FIREBALL_H
 
+#include <physfs.h>
+#include "maths.h"
+
 #ifdef __cplusplus
 extern "C" {
 #endif
 
+struct object;
+struct vms_vector;
+
 // explosion types
 #define ET_SPARKS       0   //little sparks, like when laser hits wall
 #define ET_MULTI_START  1   //first part of multi-part explosion
diff --git a/common/main/gamemine.h b/common/main/gamemine.h
index cf79ca480..869036cd8 100644
--- a/common/main/gamemine.h
+++ b/common/main/gamemine.h
@@ -21,6 +21,10 @@ COPYRIGHT 1993-1999 PARALLAX SOFTWARE CORPORATION.  ALL RIGHTS RESERVED.
 #ifndef _GAMEMINE_H
 #define _GAMEMINE_H
 
+#include <physfs.h>
+#include "maths.h"
+#include "vecmat.h"
+
 #ifdef __cplusplus
 extern "C" {
 #endif
diff --git a/common/main/gamepal.h b/common/main/gamepal.h
index 6863ad755..3eceead31 100644
--- a/common/main/gamepal.h
+++ b/common/main/gamepal.h
@@ -20,6 +20,8 @@ COPYRIGHT 1993-1999 PARALLAX SOFTWARE CORPORATION.  ALL RIGHTS RESERVED.
 #ifndef _GAMEPAL_H
 #define _GAMEPAL_H
 
+#include "inferno.h"
+
 #ifdef __cplusplus
 extern "C" {
 #endif
diff --git a/common/main/gamesave.h b/common/main/gamesave.h
index de75fa9d6..a53a0c02c 100644
--- a/common/main/gamesave.h
+++ b/common/main/gamesave.h
@@ -21,6 +21,8 @@ COPYRIGHT 1993-1999 PARALLAX SOFTWARE CORPORATION.  ALL RIGHTS RESERVED.
 #ifndef _GAMESAVE_H
 #define _GAMESAVE_H
 
+#include "pstypes.h"
+
 #ifdef __cplusplus
 extern "C" {
 #endif
diff --git a/common/main/hostage.h b/common/main/hostage.h
index 99dc55524..bd2f9fddb 100644
--- a/common/main/hostage.h
+++ b/common/main/hostage.h
@@ -25,6 +25,8 @@ COPYRIGHT 1993-1999 PARALLAX SOFTWARE CORPORATION.  ALL RIGHTS RESERVED.
 extern "C" {
 #endif
 
+struct object;
+
 #define HOSTAGE_SIZE        i2f(5)  // 3d size of a hostage
 
 #define MAX_HOSTAGE_TYPES   1       //only one hostage bitmap
diff --git a/common/main/kmatrix.h b/common/main/kmatrix.h
index c7ae2600a..0a02653c6 100644
--- a/common/main/kmatrix.h
+++ b/common/main/kmatrix.h
@@ -21,6 +21,8 @@ COPYRIGHT 1993-1999 PARALLAX SOFTWARE CORPORATION.  ALL RIGHTS RESERVED.
 #ifndef _KMATRIX_H
 #define _KMATRIX_H
 
+#include "maths.h"
+
 #ifdef __cplusplus
 extern "C" {
 #endif
diff --git a/common/main/laser.h b/common/main/laser.h
index d30449d36..75f6a7cf3 100644
--- a/common/main/laser.h
+++ b/common/main/laser.h
@@ -20,6 +20,9 @@ COPYRIGHT 1993-1999 PARALLAX SOFTWARE CORPORATION.  ALL RIGHTS RESERVED.
 #ifndef _LASER_H
 #define _LASER_H
 
+#include "maths.h"
+#include "vecmat.h"
+
 #ifdef __cplusplus
 extern "C" {
 #endif
diff --git a/common/main/lighting.h b/common/main/lighting.h
index ab0879d47..b5b78b808 100644
--- a/common/main/lighting.h
+++ b/common/main/lighting.h
@@ -21,10 +21,16 @@ COPYRIGHT 1993-1999 PARALLAX SOFTWARE CORPORATION.  ALL RIGHTS RESERVED.
 #ifndef _LIGHTING_H
 #define _LIGHTING_H
 
+#include "maths.h"
+#include "segment.h"
+
 #ifdef __cplusplus
 extern "C" {
 #endif
 
+struct g3s_lrgb;
+struct object;
+
 #define MAX_LIGHT       0x10000     // max value
 
 #define MIN_LIGHT_DIST  (F1_0*4)
diff --git a/common/main/multibot.h b/common/main/multibot.h
index a27e1f657..2a41101fd 100644
--- a/common/main/multibot.h
+++ b/common/main/multibot.h
@@ -21,10 +21,15 @@ COPYRIGHT 1993-1999 PARALLAX SOFTWARE CORPORATION.  ALL RIGHTS RESERVED.
 #ifndef _MULTIBOT_H
 #define _MULTIBOT_H
 
+#include "pstypes.h"
+
 #ifdef __cplusplus
 extern "C" {
 #endif
 
+struct object;
+struct vms_vector;
+
 #if defined(DXX_BUILD_DESCENT_I)
 #define MAX_ROBOTS_CONTROLLED 3
 #elif defined(DXX_BUILD_DESCENT_II)
diff --git a/common/main/piggy.h b/common/main/piggy.h
index 65ef51d24..ecb2f3aa6 100644
--- a/common/main/piggy.h
+++ b/common/main/piggy.h
@@ -25,6 +25,7 @@ COPYRIGHT 1993-1999 PARALLAX SOFTWARE CORPORATION.  ALL RIGHTS RESERVED.
 #include "sounds.h"
 #include "hash.h"
 #include "inferno.h"
+#include "gr.h"
 
 #ifdef __cplusplus
 extern "C" {
diff --git a/common/main/segpoint.h b/common/main/segpoint.h
index 46ca5e2cc..5edb1f0f6 100644
--- a/common/main/segpoint.h
+++ b/common/main/segpoint.h
@@ -21,7 +21,9 @@ COPYRIGHT 1993-1999 PARALLAX SOFTWARE CORPORATION.  ALL RIGHTS RESERVED.
 #ifndef _SEGPOINTS_H
 #define _SEGPOINTS_H
 
-#include "3d.h"
+#include "segment.h"
+
+struct g3s_point;
 
 extern	g3s_point	Segment_points[MAX_VERTICES];
 
diff --git a/common/main/terrain.h b/common/main/terrain.h
index bf4fc18be..f7807de01 100644
--- a/common/main/terrain.h
+++ b/common/main/terrain.h
@@ -25,6 +25,8 @@ COPYRIGHT 1993-1999 PARALLAX SOFTWARE CORPORATION.  ALL RIGHTS RESERVED.
 extern "C" {
 #endif
 
+struct vms_vector;
+
 void free_light_table();
 void free_height_array();
 void load_terrain(const char *filename);
diff --git a/common/main/texmerge.h b/common/main/texmerge.h
index d37c2390e..dd5c2059a 100644
--- a/common/main/texmerge.h
+++ b/common/main/texmerge.h
@@ -25,6 +25,8 @@ COPYRIGHT 1993-1999 PARALLAX SOFTWARE CORPORATION.  ALL RIGHTS RESERVED.
 extern "C" {
 #endif
 
+struct grs_bitmap;
+
 int texmerge_init(int num_cached_textures);
 grs_bitmap *texmerge_get_cached_bitmap(int tmap_bottom, int tmap_top);
 void texmerge_close();
diff --git a/common/main/titles.h b/common/main/titles.h
index a2e4ddbc8..70eb8fb94 100644
--- a/common/main/titles.h
+++ b/common/main/titles.h
@@ -20,6 +20,8 @@ COPYRIGHT 1993-1999 PARALLAX SOFTWARE CORPORATION.  ALL RIGHTS RESERVED.
 #ifndef _TITLES_H
 #define _TITLES_H
 
+#include "pstypes.h"
+
 #ifdef __cplusplus
 extern "C" {
 #endif
diff --git a/common/texmap/texmapl.h b/common/texmap/texmapl.h
index 13d1187ad..64ac47b99 100644
--- a/common/texmap/texmapl.h
+++ b/common/texmap/texmapl.h
@@ -20,10 +20,15 @@ COPYRIGHT 1993-1998 PARALLAX SOFTWARE CORPORATION.  ALL RIGHTS RESERVED.
 
 //	Local include file for texture map library.
 
+#include "maths.h"
+#include "pstypes.h"
+
 #ifdef __cplusplus
 extern "C" {
 #endif
 
+struct g3ds_tmap;
+
 extern	int prevmod(int val,int modulus);
 extern	int succmod(int val,int modulus);
 extern	void texture_map_flat(g3ds_tmap *t,int color);
diff --git a/d1x-rebirth/main/custom.h b/d1x-rebirth/main/custom.h
index 165f30911..1fa487bf6 100644
--- a/d1x-rebirth/main/custom.h
+++ b/d1x-rebirth/main/custom.h
@@ -1,6 +1,9 @@
 #ifndef _CUSTOM_H
 #define _CUSTOM_H
 
+#include "pstypes.h"
+#include "piggy.h"
+
 /* from piggy.c */
 #define DBM_FLAG_LARGE	128		// Flags added onto the flags struct in b
 #define DBM_FLAG_ABM            64