diff -ur csound5~/Engine/csound_orc.l csound5/Engine/csound_orc.l
--- csound5~/Engine/csound_orc.l	2011-11-22 11:11:54.000000000 +0100
+++ csound5/Engine/csound_orc.l	2011-11-22 19:27:14.000000000 +0100
@@ -49,6 +49,8 @@
 void do_macro_arg(CSOUND *, char *, yyscan_t);
 void do_macro(CSOUND *, char *, yyscan_t);
 void do_umacro(CSOUND *, char *, yyscan_t);
+void do_ifdef(CSOUND *, char *, yyscan_t);
+void do_ifdef_skip_code(CSOUND *, yyscan_t);
 #define udoflag csound->parserUdoflag
 #define namedInstrFlag csound->parserNamedInstrFlag
 //extern int udoflag;
@@ -92,6 +94,9 @@
 INCLUDE         "#include"
 DEFINE          "#define"
 UNDEF           "#undef"
+IFDEF           #ifn?def
+ELSE            #else[ \t]*(;.*)?$
+END             #end(if)?[ \t]*(;.*)?\n
 CONT            \\[[ \t]*\n
 XSTR            "{{"
 EXSTR           "}}"
@@ -99,6 +104,7 @@
 %x incl
 %x macro
 %x umacro
+%x ifdef
 %x xstr
 
 %%
@@ -495,6 +501,36 @@
                                  csound_orcget_lineno(yyscanner),
                                  yytext[0],yytext[0]);
                 }
+{IFDEF}         {
+                  PARM->isIfndef = (yytext[3] == 'n');  /* #ifdef or #ifndef */
+                  BEGIN(ifdef);
+                }
+<ifdef>[ \t]*     /* eat the whitespace */
+<ifdef>{IDENT}  {
+                  do_ifdef(csound, yytext, yyscanner);
+                  BEGIN(INITIAL);
+                }
+{ELSE}          { 
+                  if (PARM->ifdefStack == NULL) {
+                    csound->Message(csound, Str("#else without #if\n"));
+                    csound->LongJmp(csound, 1); 
+                  }
+                  else if (PARM->ifdefStack->isElse) {
+                    csound->Message(csound, Str("#else after #else\n"));
+                    csound->LongJmp(csound, 1);
+                  }
+                  PARM->ifdefStack->isElse = 1;
+                  do_ifdef_skip_code(csound, yyscanner);
+                }
+{END}           {
+                  IFDEFSTACK *pp = PARM->ifdefStack;
+                  if (UNLIKELY(pp == NULL)) {
+                    csound->Message(csound, Str("Unmatched #end\n"));
+                    csound->LongJmp(csound, 1);
+                  }
+                  PARM->ifdefStack = pp->prv;
+                  mfree(csound, pp);
+                }
 
 
 %%
@@ -667,6 +703,74 @@
     csound_orcset_lineno(1+csound_orcget_lineno(yyscanner),yyscanner);
 }
 
+void do_ifdef(CSOUND *csound, char *name0, yyscan_t yyscanner)
+{
+    int c;
+    MACRO *mm;
+    IFDEFSTACK *pp;
+    pp = (IFDEFSTACK*) mcalloc(csound, sizeof(IFDEFSTACK));
+    pp->prv = PARM->ifdefStack;
+    pp->isDef = PARM->isIfndef;
+    for (mm = PARM->macros; mm != NULL; mm = mm->next) {
+      if (strcmp(name0, mm->name) == 0) {
+        pp->isDef ^= (unsigned char) 1;
+        break;
+      }
+    }
+    PARM->ifdefStack = pp;
+    pp->isSkip = pp->isDef ^ (unsigned char) 1;
+    if (pp->isSkip)
+      do_ifdef_skip_code(csound, yyscanner);
+    else
+      while ((c = input(yyscanner)) != '\n' && c != EOF);
+}
+
+void do_ifdef_skip_code(CSOUND *csound, yyscan_t yyscanner)
+{
+    int i, c, nested_ifdef = 0;
+    char *buf;
+    IFDEFSTACK *pp;
+    buf = (char*)malloc(8*sizeof(char));
+    pp = PARM->ifdefStack;
+    c = input(yyscanner);
+    for (;;) {
+      while (c!='\n') {
+        if (UNLIKELY(c == EOF)) {
+          csound->Message(csound, Str("Unmatched #if%sdef\n"),
+                          PARM->isIfndef ? "n" : "");
+          csound->LongJmp(csound, 1);
+        }
+        c = input(yyscanner);
+      }
+      while (isblank(c = input(yyscanner)));  /* eat the whitespace */
+      if (c == '#') {
+        for (i=0; islower(c = input(yyscanner)) && i < 7; i++)
+          buf[i] = c;
+        buf[i] = '\0';
+        if (strcmp("end", buf) == 0 || strcmp("endif", buf) == 0) {
+          if (nested_ifdef-- == 0) {
+            PARM->ifdefStack = pp->prv;
+            mfree(csound, pp);
+            break;
+          }
+        }
+        else if (strcmp("ifdef", buf) == 0 || strcmp("ifndef", buf) == 0) {
+          nested_ifdef++;
+        }
+        else if (strcmp("else", buf) == 0 && nested_ifdef == 0) {
+          if (pp->isElse) {
+            csound->Message(csound, Str("#else after #else\n"));
+            csound->LongJmp(csound, 1);
+          }
+          pp->isElse = 1;
+          break;
+        }
+      }
+    }
+    free(buf);
+    while (c != '\n' && c != EOF) c = input(yyscanner);
+}
+
 ORCTOKEN *new_token(CSOUND *csound, int type)
 {
     ORCTOKEN *ans = (ORCTOKEN*)mcalloc(csound, sizeof(ORCTOKEN));
diff -ur csound5~/Engine/new_orc_parser.c csound5/Engine/new_orc_parser.c
--- csound5~/Engine/new_orc_parser.c	2011-11-20 14:38:05.000000000 +0100
+++ csound5/Engine/new_orc_parser.c	2011-11-23 09:52:49.000000000 +0100
@@ -82,6 +82,10 @@
 
     retVal = csound_orcparse(&pp, pp.yyscanner, csound, astTree);
 
+    if (UNLIKELY(pp.ifdefStack != NULL)) {
+      csound->Message(csound, Str("Unmatched #ifdef\n"));
+      csound->LongJmp(csound, 1);
+    }
     if (LIKELY(retVal == 0)) {
       csound->Message(csound, "Parsing successful!\n");
     }
diff -ur csound5~/Engine/parse_param.h csound5/Engine/parse_param.h
--- csound5~/Engine/parse_param.h	2011-11-22 11:11:54.000000000 +0100
+++ csound5/Engine/parse_param.h	2011-11-22 19:27:27.000000000 +0100
@@ -16,6 +16,15 @@
     char          *arg[MARGS];  /* With these arguments */
 } MACRO;
 
+typedef struct IFDEFSTACK_ {
+    struct IFDEFSTACK_  *prv;
+    unsigned char   isDef;      /* non-zero if #ifdef is true, or #ifndef   */
+                                /*   is false                               */
+    unsigned char   isElse;     /* non-zero between #else and #endif        */
+    unsigned char   isSkip;     /* sum of: 1: skipping code due to this     */
+                                /*   #ifdef, 2: skipping due to parent      */
+} IFDEFSTACK;
+
 typedef struct parse_parm_s {
     void            *yyscanner;
     char            *buffer;
@@ -30,6 +39,8 @@
     unsigned int macro_stack_ptr;
     char            *xstrbuff;
     int             xstrptr,xstrmax;
+    IFDEFSTACK      *ifdefStack;
+    unsigned char   isIfndef;
     unsigned char   isInclude;
     unsigned char   clearBufferAfterEOF;
 } PARSE_PARM;