diff --git a/httpie/downloads.py b/httpie/downloads.py
index fe9b900e..c1ed6574 100644
--- a/httpie/downloads.py
+++ b/httpie/downloads.py
@@ -3,10 +3,11 @@ Download mode implementation.
 
 """
 from __future__ import division
-import mimetypes
 import os
+import re
 import sys
 import errno
+import mimetypes
 from time import time
 
 from .output import RawStream
@@ -14,6 +15,8 @@ from .models import HTTPResponse
 from .utils import humanize_bytes
 from .compat import urlsplit
 
+PARTIAL_CONTENT = 206
+
 
 class Download(object):
 
@@ -37,16 +40,16 @@ class Download(object):
         self._progress = Progress(output=progress_file)
         self._resumed_from = 0
 
-    def pre_request(self, headers):
+    def pre_request(self, request_headers):
         """Called just before the HTTP request is sent.
 
-        Might alter `headers`.
+        Might alter `request_headers`.
 
-        :type headers: dict
+        :type request_headers: dict
 
         """
         # Disable content encoding so that we can resume, etc.
-        headers['Accept-Encoding'] = ''
+        request_headers['Accept-Encoding'] = ''
         if self._resume:
             try:
                 bytes_have = os.path.getsize(self._output_file.name)
@@ -56,8 +59,7 @@ class Download(object):
             else:
                 self._resumed_from = bytes_have
                 # Set ``Range`` header to resume the download
-                # TODO: What if Range isn't supported by the server?
-                headers['Range'] = '%d-' % bytes_have
+                request_headers['Range'] = 'bytes=%d-' % bytes_have
 
     def start(self, response):
         """
@@ -72,12 +74,25 @@ class Download(object):
         """
         assert not self._progress._time_started
 
-        content_length = response.headers.get('Content-Length')
-        if content_length:
-            content_length = int(content_length)
+        total_size = response.headers.get('Content-Length')
+        if total_size:
+            total_size = int(total_size)
 
         if self._output_file:
-            if not self._resume:
+            if self._resume and response.status_code == PARTIAL_CONTENT:
+                # "Content-Range: bytes 21010-47021/47022"
+                content_range = response.headers.get('Content-Range', '')
+                pattern = '^bytes {have:d}-\d+/(\*|\d+)$'.format(
+                    have=self._resumed_from)
+                match = re.match(pattern, content_range)
+                if not match:
+                    raise ValueError(
+                        'The server returned invalid Content-Range: %s'
+                        % content_range
+                    )
+                total_size += self._resumed_from
+            else:
+                self._resumed_from = 0
                 self._output_file.seek(0)
                 self._output_file.truncate()
         else:
@@ -94,7 +109,7 @@ class Download(object):
 
         self._progress.started(
             resumed_from=self._resumed_from,
-            content_length=content_length
+            total_size=total_size
         )
 
         stream = RawStream(
@@ -121,8 +136,8 @@ class Download(object):
     def interrupted(self):
         return (
             self._output_file.closed
-            and self._progress.content_length
-            and self._progress.content_length != self._progress.downloaded
+            and self._progress.total_size
+            and self._progress.total_size != self._progress.downloaded
         )
 
     def _on_progress(self, chunk):
@@ -175,7 +190,7 @@ class Progress(object):
         """
         self.output = output
         self.downloaded = 0
-        self.content_length = None
+        self.total_size = None
         self._resumed_from = 0
         self._downloaded_prev = 0
         self._content_length_humanized = '?'
@@ -184,11 +199,11 @@ class Progress(object):
         self._time_prev = None
         self._speed = 0
 
-    def started(self, resumed_from=0, content_length=None):
+    def started(self, resumed_from=0, total_size=None):
         assert self._time_started is None
-        if content_length is not None:
-            self._content_length_humanized = humanize_bytes(content_length)
-            self.content_length = content_length
+        if total_size is not None:
+            self._content_length_humanized = humanize_bytes(total_size)
+            self.total_size = total_size
         self.downloaded = self._resumed_from = resumed_from
         self._time_started = time()
         self._time_prev = self._time_started
@@ -210,9 +225,9 @@ class Progress(object):
         self._time_prev = now
         self._downloaded_prev = self.downloaded
 
-        if self.content_length:
+        if self.total_size:
             template = self.PROGRESS
-            percentage = self.downloaded / self.content_length * 100
+            percentage = self.downloaded / self.total_size * 100
         else:
             template = self.PROGRESS_NO_CONTENT_LENGTH
             percentage = None