diff --git a/Makefile b/Makefile
index a87a1fcce05e8725978e2e158c53a028138edb77..9993e004153b0de2bb22f70465b3bf6e74d047d9 100644
--- a/Makefile
+++ b/Makefile
@@ -14,7 +14,7 @@ chibicc: $(OBJS)
 $(OBJS): chibicc.h
 
 test/%.exe: chibicc test/%.c
-	./chibicc -c -o test/$*.o test/$*.c
+	./chibicc -Itest -c -o test/$*.o test/$*.c
 	$(CC) -o $@ test/$*.o -xc test/common
 
 test: $(TESTS)
@@ -35,7 +35,7 @@ stage2/%.o: chibicc self.py %.c
 
 stage2/test/%.exe: stage2/chibicc test/%.c
 	mkdir -p stage2/test
-	./stage2/chibicc -c -o stage2/test/$*.o test/$*.c
+	./stage2/chibicc -Itest -c -o stage2/test/$*.o test/$*.c
 	$(CC) -o $@ stage2/test/$*.o -xc test/common
 
 test-stage2: $(TESTS:test/%=stage2/test/%)
diff --git a/chibicc.h b/chibicc.h
index 9cd1ed26b822372f6b2e45df182749d44fa94f9b..4a09b1954bc7811d39a6fca242dec85a878d6751 100644
--- a/chibicc.h
+++ b/chibicc.h
@@ -348,4 +348,5 @@ int align_to(int n, int align);
 
 bool file_exists(char *path);
 
+extern StringArray include_paths;
 extern char *base_file;
diff --git a/main.c b/main.c
index 4c520c5d0d9a90f51f61b4150917babf3c7e94c7..4abfeca9a9a3cba0c97b56655f0240957fde9925 100644
--- a/main.c
+++ b/main.c
@@ -1,5 +1,7 @@
 #include "chibicc.h"
 
+StringArray include_paths;
+
 static bool opt_E;
 static bool opt_S;
 static bool opt_c;
@@ -19,7 +21,12 @@ static void usage(int status) {
 }
 
 static bool take_arg(char *arg) {
-  return !strcmp(arg, "-o");
+  char *x[] = {"-o", "-I"};
+
+  for (int i = 0; i < sizeof(x) / sizeof(*x); i++)
+    if (!strcmp(arg, x[i]))
+      return true;
+  return false;
 }
 
 static void parse_args(int argc, char **argv) {
@@ -69,6 +76,11 @@ static void parse_args(int argc, char **argv) {
       continue;
     }
 
+    if (!strncmp(argv[i], "-I", 2)) {
+      strarray_push(&include_paths, argv[i] + 2);
+      continue;
+    }
+
     if (!strcmp(argv[i], "-cc1-input")) {
       base_file = argv[++i];
       continue;
diff --git a/preprocess.c b/preprocess.c
index ca06ed91e4af7cf52dc83af8c2cedf0e5f9259a2..487e5b495a1ad5cb8c317fc710288d4c37dedf89 100644
--- a/preprocess.c
+++ b/preprocess.c
@@ -596,6 +596,19 @@ static bool expand_macro(Token **rest, Token *tok) {
   return true;
 }
 
+static char *search_include_paths(char *filename) {
+  if (filename[0] == '/')
+    return filename;
+
+  // Search a file from the include paths.
+  for (int i = 0; i < include_paths.len; i++) {
+    char *path = format("%s/%s", include_paths.data[i], filename);
+    if (file_exists(path))
+      return path;
+  }
+  return NULL;
+}
+
 // Read an #include argument.
 static char *read_include_filename(Token **rest, Token *tok, bool *is_dquote) {
   // Pattern 1: #include "foo.h"
@@ -669,7 +682,7 @@ static Token *preprocess2(Token *tok) {
       bool is_dquote;
       char *filename = read_include_filename(&tok, tok->next, &is_dquote);
 
-      if (filename[0] != '/') {
+      if (filename[0] != '/' && is_dquote) {
         char *path = format("%s/%s", dirname(strdup(start->file->name)), filename);
         if (file_exists(path)) {
           tok = include_file(tok, path, start->next->next);
@@ -677,8 +690,8 @@ static Token *preprocess2(Token *tok) {
         }
       }
 
-      // TODO: Search a file from the include paths.
-      tok = include_file(tok, filename, start->next->next);
+      char *path = search_include_paths(filename);
+      tok = include_file(tok, path ? path : filename, start->next->next);
       continue;
     }
 
diff --git a/self.py b/self.py
index 32e058071534e4611a44745bd566b956338de891..88af153fdeedee53873d5ec340c45f9035fc61ac 100755
--- a/self.py
+++ b/self.py
@@ -95,6 +95,8 @@ FILE *open_memstream(char **ptr, size_t *sizeloc);
 char *dirname(char *path);
 char *strncpy(char *dest, char *src, long n);
 int stat(char *pathname, struct stat *statbuf);
+int stat(char *pathname, struct stat *statbuf);
+char *dirname(char *path);
 """)
 
 for path in sys.argv[1:]:
diff --git a/test/driver.sh b/test/driver.sh
index 2358af8895dfa3ed689fb8a1368699178634751a..80bc0db790019fb7d230b6a53595e3fb21796d08 100755
--- a/test/driver.sh
+++ b/test/driver.sh
@@ -85,4 +85,10 @@ echo "#include \"$tmp/out1\"" | $chibicc -E -o $tmp/out2 -
 cat $tmp/out2 | grep -q foo
 check '-E and -o'
 
+# -I
+mkdir $tmp/dir
+echo foo > $tmp/dir/i-option-test
+echo "#include \"i-option-test\"" | $chibicc -I$tmp/dir -E - | grep -q foo
+check -I
+
 echo OK