diff --git a/remarshal.py b/remarshal.py
index 41aef75..3ca7e65 100755
--- a/remarshal.py
+++ b/remarshal.py
@@ -81,6 +81,17 @@ def argv0_to_format(argv0):
         return False, None, None
 
 
+def extension_to_format(path):
+    _, ext = os.path.splitext(path)
+
+    ext = ext[1:]
+
+    if ext == 'yml':
+        ext = 'yaml'
+
+    return ext if ext in FORMATS else None
+
+
 def json_serialize(obj):
     if isinstance(obj, datetime.datetime):
         return obj.isoformat()
@@ -119,12 +130,10 @@ def parse_command_line(argv):
     if not format_from_argv0:
         parser.add_argument('--if', '-if', '--input-format',
                             dest='input_format',
-                            required=True,
                             help="input format",
                             choices=FORMATS)
         parser.add_argument('--of', '-of', '--output-format',
                             dest='output_format',
-                            required=True,
                             help="output format",
                             choices=FORMATS)
 
@@ -177,6 +186,16 @@ def parse_command_line(argv):
             args.__dict__['indent_json'] = None
         if argv0_to != 'yaml':
             args.__dict__['yaml_style'] = None
+    else:
+        if args.input_format is None:
+            args.input_format = extension_to_format(args.input)
+            if args.input_format is None:
+                parser.error('Cannot determine the input format')
+
+        if args.output_format is None:
+            args.output_format = extension_to_format(args.output)
+            if args.output_format is None:
+                parser.error('Cannot determine the output format')
 
     # Wrap yaml_style.
     args.__dict__['yaml_options'] = {'default_style': args.yaml_style}
diff --git a/tests/test_remarshal.py b/tests/test_remarshal.py
index 9418fc1..5d9ecda 100755
--- a/tests/test_remarshal.py
+++ b/tests/test_remarshal.py
@@ -208,6 +208,44 @@ class TestRemarshal(unittest.TestCase):
         test_format_string('{0}2{1}.exe')
         test_format_string('{0}2{1}-script.py')
 
+    def test_format_detection(self):
+        ext_to_fmt = {
+            'json': 'json',
+            'toml': 'toml',
+            'yaml': 'yaml',
+            'yml': 'yaml',
+        }
+
+        for from_ext in ext_to_fmt.keys():
+            for to_ext in ext_to_fmt.keys():
+                args = remarshal.parse_command_line([
+                    sys.argv[0],
+                    'input.' + from_ext,
+                    'output.' + to_ext
+                ])
+
+                self.assertEqual(args.input_format, ext_to_fmt[from_ext])
+                self.assertEqual(args.output_format, ext_to_fmt[to_ext])
+
+    def test_format_detection_failure_input_stdin(self):
+        with self.assertRaises(SystemExit) as cm:
+            remarshal.parse_command_line([sys.argv[0], '-'])
+        self.assertEqual(cm.exception.code, 2)
+
+    def test_format_detection_failure_input_txt(self):
+        with self.assertRaises(SystemExit) as cm:
+            remarshal.parse_command_line([sys.argv[0], 'input.txt'])
+        self.assertEqual(cm.exception.code, 2)
+
+    def test_format_detection_failure_output_txt(self):
+        with self.assertRaises(SystemExit) as cm:
+            remarshal.parse_command_line([
+                sys.argv[0],
+                'input.json',
+                'output.txt'
+            ])
+        self.assertEqual(cm.exception.code, 2)
+
     def test_run_no_args(self):
         with self.assertRaises(SystemExit) as cm:
             remarshal.run([sys.argv[0]])
@@ -233,6 +271,11 @@ class TestRemarshal(unittest.TestCase):
             remarshal.run(args)
         self.assertEqual(cm.exception.errno, errno.ENOENT)
 
+    def test_run_no_output_format(self):
+        with self.assertRaises(SystemExit) as cm:
+            remarshal.run([sys.argv[0], test_file_path('array.toml')])
+        self.assertEqual(cm.exception.code, 2)
+
     def test_ordered_simple(self):
         for from_ in 'json', 'toml', 'yaml':
             for to in 'json', 'toml', 'yaml':