diff --git a/collects/lang/htdp-advanced.rkt b/collects/lang/htdp-advanced.rkt
index 3b5a4ff5c5..0673617153 100644
--- a/collects/lang/htdp-advanced.rkt
+++ b/collects/lang/htdp-advanced.rkt
@@ -1,10 +1,10 @@
 (module htdp-advanced scheme/base
-  (require "private/teach.ss"
-	   "private/teach-module-begin.ss"
-	   mzlib/etc
-	   mzlib/list
-	   mzlib/pretty
-	   syntax/docprovide
+  (require "private/teach.rkt"
+           "private/teach-module-begin.rkt"
+           mzlib/etc
+           mzlib/list
+           mzlib/pretty
+           syntax/docprovide
            scheme/promise
            test-engine/scheme-tests
            "posn.rkt")
diff --git a/collects/lang/htdp-beginner-abbr.rkt b/collects/lang/htdp-beginner-abbr.rkt
index 762c78b93e..cb66a54676 100644
--- a/collects/lang/htdp-beginner-abbr.rkt
+++ b/collects/lang/htdp-beginner-abbr.rkt
@@ -6,8 +6,8 @@
            test-engine/scheme-tests)
 
   ;; Implements the forms:
-  (require "private/teach.ss"
-	   "private/teach-module-begin.ss")
+  (require "private/teach.rkt"
+           "private/teach-module-begin.rkt")
 
   ;; syntax:
   (provide (rename-out
diff --git a/collects/lang/htdp-beginner.rkt b/collects/lang/htdp-beginner.rkt
index 92d54af6be..b294fcf0d1 100644
--- a/collects/lang/htdp-beginner.rkt
+++ b/collects/lang/htdp-beginner.rkt
@@ -4,15 +4,15 @@
 
 (module htdp-beginner scheme/base
   (require mzlib/etc
-	   mzlib/list
-	   syntax/docprovide
+           mzlib/list
+           syntax/docprovide
            "private/rewrite-error-message.rkt"
            (for-syntax "private/rewrite-error-message.rkt")
            (for-syntax scheme/base))
 
   ;; Implements the forms:
-  (require "private/teach.ss"
-           "private/teach-module-begin.ss"
+  (require "private/teach.rkt"
+           "private/teach-module-begin.rkt"
            test-engine/scheme-tests)
 
   ;; syntax:
@@ -54,9 +54,9 @@
 ; 	   Property
 ; 	   check-property for-all ==> expect expect-within expect-member-of expect-range
 	   )
-  
-  (require (for-syntax "private/firstorder.ss"))
-    
+
+  (require (for-syntax "private/firstorder.rkt"))
+
 
   (define-syntax (in-rator-position-only stx)
     (syntax-case stx ()
diff --git a/collects/lang/htdp-intermediate.rkt b/collects/lang/htdp-intermediate.rkt
index d810926f6b..8f460ce84a 100644
--- a/collects/lang/htdp-intermediate.rkt
+++ b/collects/lang/htdp-intermediate.rkt
@@ -1,11 +1,11 @@
 (module htdp-intermediate scheme/base
 
-  (require "private/teach.ss"
-           "private/teachprims.ss"
-	   "private/teach-module-begin.ss"
-	   mzlib/etc
-	   mzlib/list
-	   syntax/docprovide
+  (require "private/teach.rkt"
+           "private/teachprims.rkt"
+           "private/teach-module-begin.rkt"
+           mzlib/etc
+           mzlib/list
+           syntax/docprovide
            test-engine/scheme-tests
            (for-syntax scheme/base))
 
diff --git a/collects/lang/htdp-langs.rkt b/collects/lang/htdp-langs.rkt
index b0ad32daaa..8bf2d11c0a 100644
--- a/collects/lang/htdp-langs.rkt
+++ b/collects/lang/htdp-langs.rkt
@@ -23,7 +23,7 @@
 
          ;; this module is shared between the drscheme's namespace (so loaded here) 
          ;; and the user's namespace in the teaching languages
-         "private/set-result.ss"
+         "private/set-result.rkt"
          "private/rewrite-error-message.rkt"
 
          "private/continuation-mark-key.rkt"
diff --git a/collects/lang/private/beginner-funs.rkt b/collects/lang/private/beginner-funs.rkt
index 1458331b8b..2a16b5ac3e 100644
--- a/collects/lang/private/beginner-funs.rkt
+++ b/collects/lang/private/beginner-funs.rkt
@@ -2,9 +2,9 @@
   (require mzlib/etc mzlib/list mzlib/math syntax/docprovide)
 
   ;; Implements the procedures:
-  (require "teachprims.ss"
-           "../posn.ss"
-           "../imageeq.ss")
+  (require "teachprims.rkt"
+           "../posn.rkt"
+           "../imageeq.rkt")
 
   ;; procedures with documentation:
   (provide-and-document
diff --git a/collects/lang/private/rewrite-error-message.rkt b/collects/lang/private/rewrite-error-message.rkt
index 3d8c3f7600..514ac3ebe5 100755
--- a/collects/lang/private/rewrite-error-message.rkt
+++ b/collects/lang/private/rewrite-error-message.rkt
@@ -1,8 +1,8 @@
 #lang scheme/base
 
-(require mzlib/etc 
+(require mzlib/etc
          mzlib/list
-         (for-syntax "firstorder.ss"
+         (for-syntax "firstorder.rkt"
                      scheme/base))
 
 (provide rewrite-contract-error-message
@@ -21,7 +21,6 @@
 
 (define (exn-needs-rewriting? exn)
   (exn:fail:contract? exn))
-  
 
 (define (ensure-number n-or-str)
   (if (string? n-or-str) (string->number n-or-str) n-or-str))
diff --git a/collects/lang/private/teach.rkt b/collects/lang/private/teach.rkt
index 4c6988b391..473c07ab8b 100644
--- a/collects/lang/private/teach.rkt
+++ b/collects/lang/private/teach.rkt
@@ -56,18 +56,18 @@
 
   (require "rewrite-error-message.rkt")
 
-  (require-for-syntax "teachhelp.ss"
+  (require-for-syntax "teachhelp.rkt"
                       "rewrite-error-message.rkt"
-                      "teach-shared.ss"
+                      "teach-shared.rkt"
                       "rewrite-error-message.rkt"
-		      syntax/kerncase
-		      syntax/stx
-		      syntax/struct
-		      syntax/context
-		      mzlib/include
-		      scheme/list
-		      (rename racket/base racket:define-struct define-struct)
-		      (only racket/base syntax->datum datum->syntax)
+                      syntax/kerncase
+                      syntax/stx
+                      syntax/struct
+                      syntax/context
+                      mzlib/include
+                      scheme/list
+                      (rename racket/base racket:define-struct define-struct)
+                      (only racket/base syntax->datum datum->syntax)
                       (rename racket/base kw-app #%app)
                       racket/struct-info
                       stepper/private/shared
diff --git a/collects/lang/private/teachhelp.rkt b/collects/lang/private/teachhelp.rkt
index dddb7c509e..01d4c22be3 100644
--- a/collects/lang/private/teachhelp.rkt
+++ b/collects/lang/private/teachhelp.rkt
@@ -1,13 +1,13 @@
 (module teachhelp mzscheme
-  (require "firstorder.ss"
+  (require "firstorder.rkt"
            "rewrite-error-message.rkt"
            stepper/private/shared)
 
   (require-for-syntax stepper/private/shared)
-  
-  (provide make-undefined-check 
-	   make-first-order-function)
-  
+
+  (provide make-undefined-check
+           make-first-order-function)
+
   (define (make-undefined-check check-proc tmp-id)
     (let ([set!-stx (datum->syntax-object check-proc 'set!)])
       (make-set!-transformer
diff --git a/collects/scribblings/htdp-langs/advanced.scrbl b/collects/scribblings/htdp-langs/advanced.scrbl
index 223b4b2439..d9bfb83da0 100644
--- a/collects/scribblings/htdp-langs/advanced.scrbl
+++ b/collects/scribblings/htdp-langs/advanced.scrbl
@@ -317,5 +317,4 @@ Like @racket[when], but the @racket[body-expression] is evaluated when the
 
 @section[#:tag "advanced-pre-defined"]{Pre-Defined Functions}
 
-@prim-op-defns['(lib "htdp-advanced.ss" "lang") #'here '()]
-
+@prim-op-defns['(lib "htdp-advanced.rkt" "lang") #'here '()]
diff --git a/collects/scribblings/htdp-langs/beginner-abbr.scrbl b/collects/scribblings/htdp-langs/beginner-abbr.scrbl
index efdc520f89..341404bb15 100644
--- a/collects/scribblings/htdp-langs/beginner-abbr.scrbl
+++ b/collects/scribblings/htdp-langs/beginner-abbr.scrbl
@@ -136,5 +136,4 @@ also be written with @racket[unquote-splicing].}
 
 @section[#:tag "beginner-abbr-pre-defined"]{Pre-defined Functions}
 
-@prim-op-defns['(lib "htdp-beginner-abbr.ss" "lang") #'here '()]
-
+@prim-op-defns['(lib "htdp-beginner-abbr.rkt" "lang") #'here '()]
diff --git a/collects/scribblings/htdp-langs/intermediate-lambda.scrbl b/collects/scribblings/htdp-langs/intermediate-lambda.scrbl
index aea5943abf..b8f8a72b48 100644
--- a/collects/scribblings/htdp-langs/intermediate-lambda.scrbl
+++ b/collects/scribblings/htdp-langs/intermediate-lambda.scrbl
@@ -113,5 +113,4 @@ the function.}
 
 @section[#:tag "intm-w-lambda-pre-defined"]{Pre-defined Functions}
 
-@prim-op-defns['(lib "htdp-intermediate-lambda.ss" "lang") #'here '()]
-
+@prim-op-defns['(lib "htdp-intermediate-lambda.rkt" "lang") #'here '()]
diff --git a/collects/scribblings/htdp-langs/prim-ops.rkt b/collects/scribblings/htdp-langs/prim-ops.rkt
index e078e44913..c0ab25bf68 100644
--- a/collects/scribblings/htdp-langs/prim-ops.rkt
+++ b/collects/scribblings/htdp-langs/prim-ops.rkt
@@ -1,6 +1,6 @@
 #reader scribble/reader
 #lang racket/base
-(require "common.ss"
+(require "common.rkt"
          scribble/decode
          scribble/struct
          scribble/racket
diff --git a/collects/scribblings/htdp-langs/std-grammar.rkt b/collects/scribblings/htdp-langs/std-grammar.rkt
index 0ee065a1a4..4295d1fb23 100644
--- a/collects/scribblings/htdp-langs/std-grammar.rkt
+++ b/collects/scribblings/htdp-langs/std-grammar.rkt
@@ -1,6 +1,6 @@
 #reader scribble/reader
 #lang racket/base
-(require "common.ss"
+(require "common.rkt"
          scribble/decode)
 
 (provide prim-nonterms
@@ -18,8 +18,8 @@
    form ...
    [test-case @#,racket[(check-expect expression expression)]
               @#,racket[(check-within expression expression expression)]
-	      @#,racket[(check-member-of expression expression (... ...))]
-	      @#,racket[(check-range expression expression expression)]
+              @#,racket[(check-member-of expression expression (... ...))]
+              @#,racket[(check-range expression expression expression)]
               @#,racket[(check-error expression expression)]
               @#,racket[(check-error expression)]]
    (...