diff --git a/ChangeLog b/ChangeLog
index 5f222f87c..1ea5c3265 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,22 @@
 2004-08-09  Alexandre Duret-Lutz  <adl@src.lip6.fr>
 
+	* iface/gspn/common.cc, iface/gspn/common.hh,
+	src/ltlast/atomic_prop.cc, src/ltlast/atomic_prop.hh,
+	src/ltlparse/fmterror.cc, src/ltlparse/public.hh,
+	src/ltlvisit/dotty.cc, src/ltlvisit/dotty.hh,
+	src/ltlvisit/dump.cc, src/ltlvisit/dump.hh,
+	src/ltlvisit/tostring.cc, src/ltlvisit/tostring.hh,
+	src/misc/escape.cc, src/misc/escape.hh, src/tgba/bdddict.cc,
+	src/tgba/bdddict.hh, src/tgba/bddprint.cc, src/tgba/bddprint.hh,
+	src/tgbaalgos/dotty.cc, src/tgbaalgos/dotty.hh,
+	src/tgbaalgos/lbtt.cc, src/tgbaalgos/lbtt.hh,
+	src/tgbaalgos/neverclaim.cc, src/tgbaalgos/neverclaim.hh,
+	src/tgbaalgos/save.cc, src/tgbaalgos/save.hh,
+	src/tgbaalgos/gtec/status.cc, src/tgbaalgos/gtec/status.hh,
+	src/tgbaparse/fmterror.cc, src/tgbaparse/public.hh: Include <iosfwd>
+	in headers, and prefer <ostream> in the body whenever possible.
+	* src/sanity/style.test, HACKING: Check and document this.
+
 	* src/ltlast/allnodes.hh, src/ltlast/atomic_prop.hh,
 	src/ltlast/binop.hh, src/ltlast/constant.hh,
 	src/ltlast/formula.hh, src/ltlast/multop.hh,
diff --git a/HACKING b/HACKING
index 74bc6e70f..c3fdd16fa 100644
--- a/HACKING
+++ b/HACKING
@@ -243,3 +243,8 @@ Other style recommandations
 
   * Do not systematically initialise local variables with 0 or other
     meaningless values.  This hides errors to valgrind.
+
+  * Avoid <iostream>, <ostream>, etc. in headers whenever possible.
+    Prefer <iosfwd> when predeclarations are sufficient, and then
+    use for instance use just <ostream> in the corresponding .cc file.
+    (A plain <iostream> is needed when using std::cout, std::cerr, etc.)
diff --git a/iface/gspn/common.cc b/iface/gspn/common.cc
index ae025b201..5a4862967 100644
--- a/iface/gspn/common.cc
+++ b/iface/gspn/common.cc
@@ -20,7 +20,7 @@
 // 02111-1307, USA.
 
 #include "common.hh"
-#include "ltlvisit/destroy.hh"
+#include <ostream>
 
 namespace spot
 {
diff --git a/iface/gspn/common.hh b/iface/gspn/common.hh
index 4acd216e2..ea3c582eb 100644
--- a/iface/gspn/common.hh
+++ b/iface/gspn/common.hh
@@ -23,7 +23,7 @@
 # define SPOT_IFACE_GSPN_COMMON_HH
 
 # include <string>
-# include <iostream>
+# include <iosfwd>
 
 // Do not include gspnlib.h here, or it will polute the user's
 // namespace with internal C symbols.
diff --git a/src/ltlast/atomic_prop.cc b/src/ltlast/atomic_prop.cc
index 61fb0fa06..d37793f1d 100644
--- a/src/ltlast/atomic_prop.cc
+++ b/src/ltlast/atomic_prop.cc
@@ -22,6 +22,7 @@
 #include "atomic_prop.hh"
 #include "visitor.hh"
 #include <cassert>
+#include <ostream>
 
 namespace spot
 {
diff --git a/src/ltlast/atomic_prop.hh b/src/ltlast/atomic_prop.hh
index a54117ffb..5cc945353 100644
--- a/src/ltlast/atomic_prop.hh
+++ b/src/ltlast/atomic_prop.hh
@@ -25,7 +25,7 @@
 # define SPOT_LTLAST_ATOMIC_PROP_HH
 
 #include <string>
-#include <iostream>
+#include <iosfwd>
 #include <map>
 #include "refformula.hh"
 #include "ltlenv/environment.hh"
diff --git a/src/ltlparse/fmterror.cc b/src/ltlparse/fmterror.cc
index 76f71fa66..bf4b396d8 100644
--- a/src/ltlparse/fmterror.cc
+++ b/src/ltlparse/fmterror.cc
@@ -1,4 +1,4 @@
-// Copyright (C) 2003  Laboratoire d'Informatique de Paris 6 (LIP6),
+// Copyright (C) 2003, 2004  Laboratoire d'Informatique de Paris 6 (LIP6),
 // département Systèmes Répartis Coopératifs (SRC), Université Pierre
 // et Marie Curie.
 //
@@ -20,13 +20,14 @@
 // 02111-1307, USA.
 
 #include "public.hh"
+#include <ostream>
 
-namespace spot 
+namespace spot
 {
   namespace ltl
   {
 
-    bool 
+    bool
     format_parse_errors(std::ostream& os,
 			const std::string& ltl_string,
 			parse_error_list& error_list)
diff --git a/src/ltlparse/public.hh b/src/ltlparse/public.hh
index 587ffa8a5..5186bf0f1 100644
--- a/src/ltlparse/public.hh
+++ b/src/ltlparse/public.hh
@@ -1,4 +1,4 @@
-// Copyright (C) 2003  Laboratoire d'Informatique de Paris 6 (LIP6),
+// Copyright (C) 2003, 2004  Laboratoire d'Informatique de Paris 6 (LIP6),
 // département Systèmes Répartis Coopératifs (SRC), Université Pierre
 // et Marie Curie.
 //
@@ -28,7 +28,7 @@
 # include <string>
 # include <list>
 # include <utility>
-# include <iostream>
+# include <iosfwd>
 
 namespace spot
 {
@@ -39,6 +39,7 @@ namespace spot
     /// \brief A list of parser diagnostics, as filled by parse.
     typedef std::list<parse_error> parse_error_list;
 
+
     /// \brief Build a formula from an LTL string.
     /// \param ltl_string The string to parse.
     /// \param error_list A list that will be filled with
@@ -59,7 +60,6 @@ namespace spot
 		   environment& env = default_environment::instance(),
 		   bool debug = false);
 
-
     /// \brief Format diagnostics produced by spot::ltl::parse.
     /// \param os Where diagnostics should be output.
     /// \param ltl_string The string that were parsed.
diff --git a/src/ltlvisit/dotty.cc b/src/ltlvisit/dotty.cc
index 0509017ed..c16e831d3 100644
--- a/src/ltlvisit/dotty.cc
+++ b/src/ltlvisit/dotty.cc
@@ -1,4 +1,4 @@
-// Copyright (C) 2003  Laboratoire d'Informatique de Paris 6 (LIP6),
+// Copyright (C) 2003, 2004  Laboratoire d'Informatique de Paris 6 (LIP6),
 // département Systèmes Répartis Coopératifs (SRC), Université Pierre
 // et Marie Curie.
 //
@@ -23,6 +23,7 @@
 #include "dotty.hh"
 #include "ltlast/visitor.hh"
 #include "ltlast/allnodes.hh"
+#include <ostream>
 
 namespace spot
 {
diff --git a/src/ltlvisit/dotty.hh b/src/ltlvisit/dotty.hh
index 9d22dbf6b..693269d05 100644
--- a/src/ltlvisit/dotty.hh
+++ b/src/ltlvisit/dotty.hh
@@ -1,4 +1,4 @@
-// Copyright (C) 2003  Laboratoire d'Informatique de Paris 6 (LIP6),
+// Copyright (C) 2003, 2004  Laboratoire d'Informatique de Paris 6 (LIP6),
 // département Systèmes Répartis Coopératifs (SRC), Université Pierre
 // et Marie Curie.
 //
@@ -23,7 +23,7 @@
 # define SPOT_LTLVISIT_DOTTY_HH
 
 #include <ltlast/formula.hh>
-#include <iostream>
+#include <iosfwd>
 
 namespace spot
 {
diff --git a/src/ltlvisit/dump.cc b/src/ltlvisit/dump.cc
index f55388b16..452bd5b5a 100644
--- a/src/ltlvisit/dump.cc
+++ b/src/ltlvisit/dump.cc
@@ -1,4 +1,4 @@
-// Copyright (C) 2003  Laboratoire d'Informatique de Paris 6 (LIP6),
+// Copyright (C) 2003, 2004  Laboratoire d'Informatique de Paris 6 (LIP6),
 // département Systèmes Répartis Coopératifs (SRC), Université Pierre
 // et Marie Curie.
 //
@@ -22,7 +22,7 @@
 #include "dump.hh"
 #include "ltlast/visitor.hh"
 #include "ltlast/allnodes.hh"
-
+#include <ostream>
 
 namespace spot
 {
@@ -32,7 +32,7 @@ namespace spot
     class dump_visitor : public const_visitor
     {
     public:
-      dump_visitor(std::ostream& os = std::cout)
+      dump_visitor(std::ostream& os)
 	: os_(os)
       {
       }
@@ -80,7 +80,7 @@ namespace spot
 	mo->nth(0)->accept(*this);
 	for (unsigned n = 1; n < max; ++n)
 	  {
-	    std::cout << ", ";
+	    os_ << ", ";
 	    mo->nth(n)->accept(*this);
 	  }
 	os_ << ")";
diff --git a/src/ltlvisit/dump.hh b/src/ltlvisit/dump.hh
index 922b8cb39..11b2aba8e 100644
--- a/src/ltlvisit/dump.hh
+++ b/src/ltlvisit/dump.hh
@@ -1,4 +1,4 @@
-// Copyright (C) 2003  Laboratoire d'Informatique de Paris 6 (LIP6),
+// Copyright (C) 2003, 2004  Laboratoire d'Informatique de Paris 6 (LIP6),
 // département Systèmes Répartis Coopératifs (SRC), Université Pierre
 // et Marie Curie.
 //
@@ -23,7 +23,7 @@
 # define SPOT_LTLVISIT_DUMP_HH
 
 #include "ltlast/formula.hh"
-#include <iostream>
+#include <iosfwd>
 
 namespace spot
 {
diff --git a/src/ltlvisit/tostring.cc b/src/ltlvisit/tostring.cc
index 876f1290f..d7dfaafc9 100644
--- a/src/ltlvisit/tostring.cc
+++ b/src/ltlvisit/tostring.cc
@@ -22,6 +22,7 @@
 #include <cassert>
 #include <sstream>
 #include <ctype.h>
+#include <ostream>
 #include "tostring.hh"
 #include "ltlast/visitor.hh"
 #include "ltlast/allnodes.hh"
@@ -56,7 +57,7 @@ namespace spot
     class to_string_visitor : public const_visitor
     {
     public:
-      to_string_visitor(std::ostream& os = std::cout)
+      to_string_visitor(std::ostream& os)
 	: os_(os), top_level_(true)
       {
       }
@@ -203,7 +204,7 @@ namespace spot
     class to_spin_string_visitor : public to_string_visitor
     {
     public:
-    to_spin_string_visitor(std::ostream& os = std::cout)
+    to_spin_string_visitor(std::ostream& os)
       : to_string_visitor(os)
       {
       }
diff --git a/src/ltlvisit/tostring.hh b/src/ltlvisit/tostring.hh
index 3b6558c20..8f6ff8e89 100644
--- a/src/ltlvisit/tostring.hh
+++ b/src/ltlvisit/tostring.hh
@@ -1,4 +1,4 @@
-// Copyright (C) 2003  Laboratoire d'Informatique de Paris 6 (LIP6),
+// Copyright (C) 2003, 2004  Laboratoire d'Informatique de Paris 6 (LIP6),
 // département Systèmes Répartis Coopératifs (SRC), Université Pierre
 // et Marie Curie.
 //
@@ -23,7 +23,7 @@
 # define SPOT_LTLVISIT_TOSTRING_HH
 
 #include <ltlast/formula.hh>
-#include <iostream>
+#include <iosfwd>
 
 namespace spot
 {
diff --git a/src/misc/escape.cc b/src/misc/escape.cc
index 6b4b4ded0..a4b1a246c 100644
--- a/src/misc/escape.cc
+++ b/src/misc/escape.cc
@@ -20,6 +20,7 @@
 // 02111-1307, USA.
 
 #include <sstream>
+#include <ostream>
 #include "escape.hh"
 
 namespace spot
diff --git a/src/misc/escape.hh b/src/misc/escape.hh
index 4ca2bd31a..f5eeae24e 100644
--- a/src/misc/escape.hh
+++ b/src/misc/escape.hh
@@ -22,7 +22,7 @@
 #ifndef SPOT_MISC_ESCAPE_HH
 # define SPOT_MISC_ESCAPE_HH
 
-# include <iostream>
+# include <iosfwd>
 # include <string>
 
 namespace spot
diff --git a/src/sanity/style.test b/src/sanity/style.test
index e1d41cbbe..f256d6179 100755
--- a/src/sanity/style.test
+++ b/src/sanity/style.test
@@ -1,3 +1,4 @@
+
 #! /bin/sh
 
 # Ensure consistent style by catching common improper constructs.
@@ -113,9 +114,21 @@ for dir in "${INCDIR-..}" "${INCDIR-..}"/../iface; do
     grep -e 'return[	 ]*[(][^(]*[)];' $tmp &&
       diag 'No useless parentheses after return.'
 
-    grep  'NULL' $tmp &&
+    grep 'NULL' $tmp &&
       diag 'Use 0 instead of NULL.  NULL is not portable.'
 
+    case $file in
+      *.hh | *.hxx)
+        if grep -e '(cout|cerr|clog)' $tmp >/dev/null; then
+          :
+	else
+          grep '#.*include.*<iostream>' $tmp &&
+            diag 'Avoid <iostream> in headers, better use <iosfwd>.'
+        fi
+        ;;
+    esac
+
+
     $fail && echo "$file" >>failures
   done
 done
diff --git a/src/tgba/bdddict.cc b/src/tgba/bdddict.cc
index a4a56969c..7cfef89fd 100644
--- a/src/tgba/bdddict.cc
+++ b/src/tgba/bdddict.cc
@@ -19,10 +19,11 @@
 // Software Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA
 // 02111-1307, USA.
 
+#include <ostream>
+#include <cassert>
 #include <ltlvisit/clone.hh>
 #include <ltlvisit/destroy.hh>
 #include <ltlvisit/tostring.hh>
-#include <cassert>
 #include "bdddict.hh"
 
 namespace spot
diff --git a/src/tgba/bdddict.hh b/src/tgba/bdddict.hh
index 691b7eb50..26332a9de 100644
--- a/src/tgba/bdddict.hh
+++ b/src/tgba/bdddict.hh
@@ -25,7 +25,7 @@
 #include "misc/hash.hh"
 #include <list>
 #include <set>
-#include <iostream>
+#include <iosfwd>
 #include <bdd.h>
 #include "ltlast/formula.hh"
 #include "misc/bddalloc.hh"
diff --git a/src/tgba/bddprint.cc b/src/tgba/bddprint.cc
index ddf89f9e5..4be59eb21 100644
--- a/src/tgba/bddprint.cc
+++ b/src/tgba/bddprint.cc
@@ -21,6 +21,7 @@
 
 #include <sstream>
 #include <cassert>
+#include <ostream>
 #include "bddprint.hh"
 #include "ltlvisit/tostring.hh"
 #include "formula2bdd.hh"
diff --git a/src/tgba/bddprint.hh b/src/tgba/bddprint.hh
index e19468daa..e8fcc3cc4 100644
--- a/src/tgba/bddprint.hh
+++ b/src/tgba/bddprint.hh
@@ -23,7 +23,7 @@
 # define SPOT_TGBA_BDDPRINT_HH
 
 #include <string>
-#include <iostream>
+#include <iosfwd>
 #include "bdddict.hh"
 #include <bdd.h>
 
diff --git a/src/tgbaalgos/dotty.cc b/src/tgbaalgos/dotty.cc
index 2e8e836a2..8b245d802 100644
--- a/src/tgbaalgos/dotty.cc
+++ b/src/tgbaalgos/dotty.cc
@@ -19,6 +19,7 @@
 // Software Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA
 // 02111-1307, USA.
 
+#include <ostream>
 #include "tgba/tgba.hh"
 #include "dotty.hh"
 #include "tgba/bddprint.hh"
diff --git a/src/tgbaalgos/dotty.hh b/src/tgbaalgos/dotty.hh
index a818dc8b7..f5143a32c 100644
--- a/src/tgbaalgos/dotty.hh
+++ b/src/tgbaalgos/dotty.hh
@@ -1,4 +1,4 @@
-// Copyright (C) 2003  Laboratoire d'Informatique de Paris 6 (LIP6),
+// Copyright (C) 2003, 2004  Laboratoire d'Informatique de Paris 6 (LIP6),
 // département Systèmes Répartis Coopératifs (SRC), Université Pierre
 // et Marie Curie.
 //
@@ -23,7 +23,7 @@
 # define SPOT_TGBAALGOS_DOTTY_HH
 
 #include "tgba/tgba.hh"
-#include <iostream>
+#include <iosfwd>
 
 namespace spot
 {
diff --git a/src/tgbaalgos/gtec/status.cc b/src/tgbaalgos/gtec/status.cc
index 0e0ae8f28..37d3f5828 100644
--- a/src/tgbaalgos/gtec/status.cc
+++ b/src/tgbaalgos/gtec/status.cc
@@ -19,6 +19,7 @@
 // Software Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA
 // 02111-1307, USA.
 
+#include <ostream>
 #include "status.hh"
 
 namespace spot
diff --git a/src/tgbaalgos/gtec/status.hh b/src/tgbaalgos/gtec/status.hh
index b0d416548..067ee5460 100644
--- a/src/tgbaalgos/gtec/status.hh
+++ b/src/tgbaalgos/gtec/status.hh
@@ -25,7 +25,7 @@
 #include "sccstack.hh"
 #include "nsheap.hh"
 #include "tgba/tgba.hh"
-#include <iostream>
+#include <iosfwd>
 
 namespace spot
 {
diff --git a/src/tgbaalgos/lbtt.cc b/src/tgbaalgos/lbtt.cc
index b6b015289..e254e6d26 100644
--- a/src/tgbaalgos/lbtt.cc
+++ b/src/tgbaalgos/lbtt.cc
@@ -22,8 +22,8 @@
 #include "lbtt.hh"
 #include <map>
 #include <string>
+#include <ostream>
 #include <sstream>
-#include <functional>
 #include "tgba/bddprint.hh"
 #include "reachiter.hh"
 #include "misc/bddlt.hh"
diff --git a/src/tgbaalgos/lbtt.hh b/src/tgbaalgos/lbtt.hh
index 4da5c2901..5c6527368 100644
--- a/src/tgbaalgos/lbtt.hh
+++ b/src/tgbaalgos/lbtt.hh
@@ -23,7 +23,7 @@
 # define SPOT_TGBAALGOS_LBTT_HH
 
 #include "tgba/tgba.hh"
-#include <iostream>
+#include <iosfwd>
 
 namespace spot
 {
diff --git a/src/tgbaalgos/neverclaim.cc b/src/tgbaalgos/neverclaim.cc
index 54a2cf3c5..74e29802e 100644
--- a/src/tgbaalgos/neverclaim.cc
+++ b/src/tgbaalgos/neverclaim.cc
@@ -19,6 +19,7 @@
 // Software Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA
 // 02111-1307, USA.
 
+#include <ostream>
 #include <sstream>
 #include "bdd.h"
 #include "tgba/tgbatba.hh"
diff --git a/src/tgbaalgos/neverclaim.hh b/src/tgbaalgos/neverclaim.hh
index ba672603e..c9aa6ef73 100644
--- a/src/tgbaalgos/neverclaim.hh
+++ b/src/tgbaalgos/neverclaim.hh
@@ -22,7 +22,7 @@
 #ifndef SPOT_TGBAALGOS_NEVERCLAIM_HH
 # define SPOT_TGBAALGOS_NEVERCLAIM_HH
 
-#include <iostream>
+#include <iosfwd>
 #include "ltlast/formula.hh"
 #include "tgba/tgbatba.hh"
 
diff --git a/src/tgbaalgos/save.cc b/src/tgbaalgos/save.cc
index 0e96806b7..cd61c797a 100644
--- a/src/tgbaalgos/save.cc
+++ b/src/tgbaalgos/save.cc
@@ -19,7 +19,7 @@
 // Software Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA
 // 02111-1307, USA.
 
-#include "tgba/tgba.hh"
+#include <ostream>
 #include "save.hh"
 #include "tgba/bddprint.hh"
 #include "ltlvisit/tostring.hh"
diff --git a/src/tgbaalgos/save.hh b/src/tgbaalgos/save.hh
index f79d750ba..88483bec2 100644
--- a/src/tgbaalgos/save.hh
+++ b/src/tgbaalgos/save.hh
@@ -1,4 +1,4 @@
-// Copyright (C) 2003  Laboratoire d'Informatique de Paris 6 (LIP6),
+// Copyright (C) 2003, 2004  Laboratoire d'Informatique de Paris 6 (LIP6),
 // département Systèmes Répartis Coopératifs (SRC), Université Pierre
 // et Marie Curie.
 //
@@ -23,7 +23,7 @@
 # define SPOT_TGBAALGOS_SAVE_HH
 
 #include "tgba/tgba.hh"
-#include <iostream>
+#include <iosfwd>
 
 namespace spot
 {
diff --git a/src/tgbaparse/fmterror.cc b/src/tgbaparse/fmterror.cc
index 4c36294bb..3d7690ae3 100644
--- a/src/tgbaparse/fmterror.cc
+++ b/src/tgbaparse/fmterror.cc
@@ -1,4 +1,4 @@
-// Copyright (C) 2003  Laboratoire d'Informatique de Paris 6 (LIP6),
+// Copyright (C) 2003, 2004  Laboratoire d'Informatique de Paris 6 (LIP6),
 // département Systèmes Répartis Coopératifs (SRC), Université Pierre
 // et Marie Curie.
 //
@@ -19,6 +19,7 @@
 // Software Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA
 // 02111-1307, USA.
 
+#include <ostream>
 #include "public.hh"
 
 namespace spot
diff --git a/src/tgbaparse/public.hh b/src/tgbaparse/public.hh
index a5d87e2cc..eb2379118 100644
--- a/src/tgbaparse/public.hh
+++ b/src/tgbaparse/public.hh
@@ -1,4 +1,4 @@
-// Copyright (C) 2003  Laboratoire d'Informatique de Paris 6 (LIP6),
+// Copyright (C) 2003, 2004  Laboratoire d'Informatique de Paris 6 (LIP6),
 // département Systèmes Répartis Coopératifs (SRC), Université Pierre
 // et Marie Curie.
 //
@@ -28,7 +28,7 @@
 # include <string>
 # include <list>
 # include <utility>
-# include <iostream>
+# include <iosfwd>
 
 namespace spot
 {