diff --git a/chibicc.h b/chibicc.h
index 00faf02..51aefe6 100644
--- a/chibicc.h
+++ b/chibicc.h
@@ -234,6 +234,7 @@ struct Node {
 };
 
 Node *new_cast(Node *expr, Type *ty);
+int64_t const_expr(Token **rest, Token *tok);
 Obj *parse(Token *tok);
 
 //
diff --git a/parse.c b/parse.c
index 9482cca..98389e3 100644
--- a/parse.c
+++ b/parse.c
@@ -129,7 +129,6 @@ static int64_t eval2(Node *node, char **label);
 static int64_t eval_rval(Node *node, char **label);
 static Node *assign(Token **rest, Token *tok);
 static Node *logor(Token **rest, Token *tok);
-static int64_t const_expr(Token **rest, Token *tok);
 static double eval_double(Node *node);
 static Node *conditional(Token **rest, Token *tok);
 static Node *logand(Token **rest, Token *tok);
@@ -1551,7 +1550,7 @@ static int64_t eval_rval(Node *node, char **label) {
   error_tok(node->tok, "invalid initializer");
 }
 
-static int64_t const_expr(Token **rest, Token *tok) {
+int64_t const_expr(Token **rest, Token *tok) {
   Node *node = conditional(rest, tok);
   return eval(node);
 }
diff --git a/preprocess.c b/preprocess.c
index a5f7316..b19b8a4 100644
--- a/preprocess.c
+++ b/preprocess.c
@@ -1,5 +1,14 @@
 #include "chibicc.h"
 
+// `#if` can be nested, so we use a stack to manage nested `#if`s.
+typedef struct CondIncl CondIncl;
+struct CondIncl {
+  CondIncl *next;
+  Token *tok;
+};
+
+static CondIncl *cond_incl;
+
 static bool is_hash(Token *tok) {
   return tok->at_bol && equal(tok, "#");
 }
@@ -22,6 +31,13 @@ static Token *copy_token(Token *tok) {
   return t;
 }
 
+static Token *new_eof(Token *tok) {
+  Token *t = copy_token(tok);
+  t->kind = TK_EOF;
+  t->len = 0;
+  return t;
+}
+
 // Append tok2 to the end of tok1.
 static Token *append(Token *tok1, Token *tok2) {
   if (!tok1 || tok1->kind == TK_EOF)
@@ -36,6 +52,54 @@ static Token *append(Token *tok1, Token *tok2) {
   return head.next;
 }
 
+// Skip until next `#endif`.
+static Token *skip_cond_incl(Token *tok) {
+  while (tok->kind != TK_EOF) {
+    if (is_hash(tok) && equal(tok->next, "endif"))
+      return tok;
+    tok = tok->next;
+  }
+  return tok;
+}
+
+// Copy all tokens until the next newline, terminate them with
+// an EOF token and then returns them. This function is used to
+// create a new list of tokens for `#if` arguments.
+static Token *copy_line(Token **rest, Token *tok) {
+  Token head = {};
+  Token *cur = &head;
+
+  for (; !tok->at_bol; tok = tok->next)
+    cur = cur->next = copy_token(tok);
+
+  cur->next = new_eof(tok);
+  *rest = tok;
+  return head.next;
+}
+
+// Read and evaluate a constant expression.
+static long eval_const_expr(Token **rest, Token *tok) {
+  Token *start = tok;
+  Token *expr = copy_line(rest, tok->next);
+
+  if (expr->kind == TK_EOF)
+    error_tok(start, "no expression");
+
+  Token *rest2;
+  long val = const_expr(&rest2, expr);
+  if (rest2->kind != TK_EOF)
+    error_tok(rest2, "extra token");
+  return val;
+}
+
+static CondIncl *push_cond_incl(Token *tok) {
+  CondIncl *ci = calloc(1, sizeof(CondIncl));
+  ci->next = cond_incl;
+  ci->tok = tok;
+  cond_incl = ci;
+  return ci;
+}
+
 // Visit all tokens in `tok` while evaluating preprocessing
 // macros and directives.
 static Token *preprocess2(Token *tok) {
@@ -50,6 +114,7 @@ static Token *preprocess2(Token *tok) {
       continue;
     }
 
+    Token *start = tok;
     tok = tok->next;
 
     if (equal(tok, "include")) {
@@ -72,6 +137,22 @@ static Token *preprocess2(Token *tok) {
       continue;
     }
 
+    if (equal(tok, "if")) {
+      long val = eval_const_expr(&tok, tok);
+      push_cond_incl(start);
+      if (!val)
+        tok = skip_cond_incl(tok);
+      continue;
+    }
+
+    if (equal(tok, "endif")) {
+      if (!cond_incl)
+        error_tok(start, "stray #endif");
+      cond_incl = cond_incl->next;
+      tok = skip_line(tok->next);
+      continue;
+    }
+
     // `#`-only line is legal. It's called a null directive.
     if (tok->at_bol)
       continue;
@@ -86,6 +167,8 @@ static Token *preprocess2(Token *tok) {
 // Entry point function of the preprocessor.
 Token *preprocess(Token *tok) {
   tok = preprocess2(tok);
+  if (cond_incl)
+    error_tok(cond_incl->tok, "unterminated conditional directive");
   convert_keywords(tok);
   return tok;
 }
diff --git a/test/macro.c b/test/macro.c
index 3777adc..dd8262b 100644
--- a/test/macro.c
+++ b/test/macro.c
@@ -14,6 +14,18 @@ int main() {
   assert(5, include1, "include1");
   assert(7, include2, "include2");
 
+#if 0
+#include "/no/such/file"
+  assert(0, 1, "1");
+#endif
+
+  int m = 0;
+
+#if 1
+  m = 5;
+#endif
+  assert(5, m, "m");
+
   printf("OK\n");
   return 0;
 }
diff --git a/tokenize.c b/tokenize.c
index 07596ec..f5f8e92 100644
--- a/tokenize.c
+++ b/tokenize.c
@@ -30,7 +30,7 @@ static void verror_at(char *filename, char *input, int line_no,
     line--;
 
   char *end = loc;
-  while (*end != '\n')
+  while (*end && *end != '\n')
     end++;
 
   // Print out the line.