diff --git a/bin/crawl-url b/bin/crawl-url
index b83d202..4b6c7d1 100755
--- a/bin/crawl-url
+++ b/bin/crawl-url
@@ -7,6 +7,8 @@ import sys
 import logging
 import umbra
 import umbra.frontier
+import threading
+import time
 
 arg_parser = argparse.ArgumentParser(prog=os.path.basename(__file__),
         description='browse-url - open urls in chrome/chromium and run behaviors',
@@ -16,6 +18,8 @@ arg_parser.add_argument('-w', '--browser-wait', dest='browser_wait', default='60
         help='seconds to wait for browser initialization')
 arg_parser.add_argument('-e', '--executable', dest='chrome_exe', default='chromium-browser',
         help='executable to use to invoke chrome')
+arg_parser.add_argument('-n', '--max-browsers', dest='max_browsers', default='1',
+        help='Max number of chrome instances simultaneously browsing pages')
 arg_parser.add_argument('-v', '--verbose', dest='log_level',
         action="store_const", default=logging.INFO, const=logging.DEBUG)
 arg_parser.add_argument('--version', action='version',
@@ -30,15 +34,23 @@ for url in args.urls:
     frontier.schedule(umbra.frontier.CrawlUrl(url, priority=1000))
 
 def frontier_schedule(urls):
-    logging.info("adding {} urls to frontier".format(len(urls)))
+    logging.info("scheduling {} urls".format(len(urls)))
     for url in urls:
         frontier.schedule(umbra.frontier.CrawlUrl(url))
 
-with umbra.Browser(chrome_exe=args.chrome_exe) as browser:
-    try:
+def crawl_from_frontier(i):
+    with umbra.Browser(chrome_port=9200 + i, chrome_exe=args.chrome_exe) as browser:
         while True:
-            crawl_url = frontier.pop()
-            browser.browse_page(crawl_url.url, on_outlinks=frontier_schedule)
-    except IndexError:
-        logging.info("finished, frontier is empty")
+            try:
+                crawl_url = frontier.pop()
+                browser.browse_page(crawl_url.url, on_outlinks=frontier_schedule)
+            except KeyError:
+                time.sleep(0.5)
 
+for i in range(int(args.max_browsers)):
+    th = threading.Thread(target=lambda: crawl_from_frontier(i), 
+            name="BrowsingThread{}".format(i))
+    th.start()
+
+while True:
+    time.sleep(0.5)
diff --git a/umbra/frontier.py b/umbra/frontier.py
index 5a713c8..4d6957b 100644
--- a/umbra/frontier.py
+++ b/umbra/frontier.py
@@ -5,6 +5,7 @@ import logging
 import sys
 import urllib.parse
 import sortedcontainers
+import threading
 
 class CrawlUrl:
     def __init__(self, url, priority=1):
@@ -29,39 +30,43 @@ class CrawlUrl:
 class Frontier:
     def __init__(self):
         # {url:CrawlUrl}
-        self.urls = {}
+        self._urls = {}
 
         # {host:SortedDict{priority_key:CrawlUrl}}
-        self.queues_by_host = {}
+        self._queues_by_host = {}
+
+        self._lock = threading.Lock()
 
     def schedule(self, crawl_url):
-        try:
-            old_priority_key = self.urls.pop(crawl_url.url).priority_key
-            old_crawl_url = self.queues_by_host[crawl_url.host].pop(old_priority_key)
+        with self._lock:
+            try:
+                old_priority_key = self._urls.pop(crawl_url.url).priority_key
+                old_crawl_url = self._queues_by_host[crawl_url.host].pop(old_priority_key)
 
-            # XXX very dumb calculation of new priority, probably doesn't belong here
-            crawl_url.set_priority(crawl_url.get_priority() + old_crawl_url.get_priority())
-        except KeyError:
-            pass
+                # XXX very dumb calculation of new priority, probably doesn't belong here
+                crawl_url.set_priority(crawl_url.get_priority() + old_crawl_url.get_priority())
+            except KeyError:
+                pass
 
-        self.urls[crawl_url.url] = crawl_url
-        if crawl_url.host not in self.queues_by_host:
-            self.queues_by_host[crawl_url.host] = sortedcontainers.SortedDict()
-        self.queues_by_host[crawl_url.host][crawl_url.priority_key] = crawl_url
+            self._urls[crawl_url.url] = crawl_url
+            if crawl_url.host not in self._queues_by_host:
+                self._queues_by_host[crawl_url.host] = sortedcontainers.SortedDict()
+            self._queues_by_host[crawl_url.host][crawl_url.priority_key] = crawl_url
 
     def pop(self, host=None):
-        if not host or host not in self.queues_by_host:
-            # XXX should prioritize queues, this picks one at random
-            for h in self.queues_by_host:
-                host = h
-                break
+        with self._lock:
+            if not host or host not in self._queues_by_host:
+                # XXX should prioritize queues, this picks one at random
+                for h in self._queues_by_host:
+                    host = h
+                    break
 
-        result = self.queues_by_host[host].popitem(last=True)[1]
-        if len(self.queues_by_host[host]) == 0:
-            del self.queues_by_host[host]
+            result = self._queues_by_host[host].popitem(last=True)[1]
+            if len(self._queues_by_host[host]) == 0:
+                del self._queues_by_host[host]
 
-        result2 = self.urls.pop(result.url)
-        assert result2 is result
+            result2 = self._urls.pop(result.url)
+            assert result2 is result
 
-        return result
+            return result