diff --git a/crawler.py b/crawler.py
index 999b23c..4cd1163 100755
--- a/crawler.py
+++ b/crawler.py
@@ -5,6 +5,7 @@ import sys
 import os
 import json
 import re
+import argparse
 
 class Error(Exception):
     pass
@@ -20,18 +21,19 @@ class UnauthorizedError(Error):
         self.extid = extid
 
 class ExtensionCrawler:
+    possible_categories = ['extensions', 'ext/22-accessibility', 'ext/10-blogging', 'ext/15-by-google', 'ext/11-web-development', 'ext/14-fun', 'ext/6-news', 'ext/28-photos', 'ext/7-productivity', 'ext/38-search-tools', 'ext/12-shopping', 'ext/1-communication', 'ext/13-sports']
     regexExtid       = re.compile(r'^[a-z]+$')
     regexExtfilename = re.compile(r'^extension[_0-9]+\.crx$')
 
-    downloadUrl      = 'https://clients2.google.com/service/update2/crx?response=redirect&nacl_arch=x86-64&prodversion=9999.0.9999.0&x=id%3D{}%26uc'
-    extensionListUrl = 'https://chrome.google.com/webstore/ajax/item?pv=20160822&count=200&category=extensions'
-    detailUrl        = 'https://chrome.google.com/webstore/detail/{}'
+    download_url      = 'https://clients2.google.com/service/update2/crx?response=redirect&nacl_arch=x86-64&prodversion=9999.0.9999.0&x=id%3D{}%26uc'
+    extension_list_url = 'https://chrome.google.com/webstore/ajax/item?pv=20160822&count={}&category={}'
+    detail_url        = 'https://chrome.google.com/webstore/detail/{}'
 
     def __init__(self, basedir):
         self.basedir = basedir
 
-    def downloadExtension(self, extid, extdir=""):
-        extresult = requests.get(self.downloadUrl.format(extid), stream=True)
+    def download_extension(self, extid, extdir=""):
+        extresult = requests.get(self.download_url.format(extid), stream=True)
         if extresult.status_code == 401:
             raise UnauthorizedError(extid)
         if not 'Content-Type' in extresult.headers:
@@ -46,12 +48,12 @@ class ExtensionCrawler:
                 if chunk: # filter out keep-alive new chunks
                     f.write(chunk)
 
-    def downloadStorepage(self, extid, extdir):
-        extpageresult = requests.get(self.detailUrl.format(extid))
+    def download_storepage(self, extid, extdir):
+        extpageresult = requests.get(self.detail_url.format(extid))
         with open(os.path.join(extdir, 'storepage.html'), 'w') as f:
             f.write(extpageresult.text)
 
-    def handleExtension(self, extinfo):
+    def handle_extension(self, extinfo):
         extid = extinfo[0]
         if not self.regexExtid.match(extid):
             raise CrawlError(extid, '{} is not a valid extension id.\n'.format(extid))
@@ -64,12 +66,13 @@ class ExtensionCrawler:
         with open(os.path.join(extdir, 'metadata.json'), 'w') as f:
             json.dump(extinfo, f, indent=5)
 
-        self.downloadStorepage(extid, extdir)
-        self.downloadExtension(extid, extdir)
+        self.download_storepage(extid, extdir)
+        self.download_extension(extid, extdir)
 
-    def run(self, nrExtensions):
+    def run(self, category, nrExtensions):
         sys.stdout.write('Downloading extensions into folder "{}"...\n'.format(self.basedir))
-        bigjson = json.loads(requests.post(self.extensionListUrl.format(nrExtensions)).text.lstrip(")]}'\n"))
+        response = requests.post(self.extension_list_url.format(nrExtensions, category)).text
+        bigjson = json.loads(response.lstrip(")]}'\n"))
         extinfos = bigjson[1][1]
 
         newExtensions = 0
@@ -78,7 +81,7 @@ class ExtensionCrawler:
             sys.stdout.write('Processing extension "{}"...'.format(extid))
             sys.stdout.flush()
             try:
-                self.handleExtension(extinfo)
+                self.handle_extension(extinfo)
                 sys.stdout.write('Done!\n')
                 newExtensions += 1
             except CrawlError as cerr:
@@ -87,11 +90,25 @@ class ExtensionCrawler:
                     sys.stderr.write('Page content was:\n')
                     sys.stderr.write('{}\n'.format(cerr.pagecontent))
             except UnauthorizedError as uerr:
-                sys.stdout.write('Error: login needed')
+                sys.stdout.write('Error: login needed\n')
         sys.stdout.write('Downloaded {} new extensions.\n'.format(newExtensions))
 
 if __name__ == '__main__':
-    crawler = ExtensionCrawler('downloaded')
+    parser = argparse.ArgumentParser(description='Downloads extensions from the Chrome Web Store.')
+    parser.add_argument('-t', '--interval', nargs='?', const=5, type=int, help='Keep downloading extensions every X seconds.')
+    parser.add_argument('-i', '--iterate', metavar='i', default=1, type=int, help='Queries the store i times for a list of extensions.')
+    parser.add_argument('-n', '--nrexts', metavar='N', default=200, type=int, help='The number of extensions to be downloaded per request (Google does not accept values much higher than 200).')
+    parser.add_argument('-c', '--category', default='extensions', choices=ExtensionCrawler.possible_categories, help='The extension category from which extensions should be downloaded.')
+    parser.add_argument('-d', '--dest', default='downloaded', help='The directory in which the downloaded extensions should be stored.')
 
-    # 200 extensions is roughly the maximum number allowed
-    crawler.run(200)
+    args = parser.parse_args()
+    crawler = ExtensionCrawler(args.dest)
+
+    if args.interval:
+        while True:
+            for i in range(args.iterate):
+                crawler.run(args.category, args.nrexts)
+                time.sleep(args.interval)
+    else:
+        for i in range(args.iterate):
+            crawler.run(args.category, args.nrexts)