diff --git a/README.rst b/README.rst
index b779f27..2719e02 100644
--- a/README.rst
+++ b/README.rst
@@ -111,6 +111,25 @@ You can throw these files up to a web server to let others view your map. You
 do *not* need a Google Maps API key (as was the case with older versions of the
 API), so just copying the directory to your web server should suffice.
 
+Clearing the Cache
+------------------
+The Overviewer keeps two levels of cache: it saves each chunk rendered
+individually along side your chunk files in your saved world directory, and it
+keeps a hash file along side each tile in your output directory. Using these
+cache files it will not re-render any image that has not changed.
+
+If you want to clear the cache and re-render everything, run gmap.py with the
+-d option::
+
+    python gmap.py -d <Path to World> <Output Directory>
+
+The next time your map is rendered, it will re-render every chunk. This is
+useful if you've changed texture packs or want to save disk space, but
+otherwise not too useful.
+
+This is probably *not* a good idea for very large worlds, since it will take
+much longer to render the next time you do so.
+
 Bugs
 ====
 This program has bugs. They are mostly minor things, I wouldn't have released a
diff --git a/gmap.py b/gmap.py
index 7165df6..2893bd6 100755
--- a/gmap.py
+++ b/gmap.py
@@ -11,7 +11,7 @@ import world
 import quadtree
 
 helptext = """
-%prog [-p PROCS] <Path to World> <tiles dest dir>
+%prog [-p PROCS] [-d] <Path to World> <tiles dest dir>
 """
 
 def main():
@@ -21,6 +21,7 @@ def main():
         cpus = 1
     parser = OptionParser(usage=helptext)
     parser.add_option("-p", "--processes", dest="procs", help="How many chunks to render in parallel. A good number for this is the number of cores in your computer. Default %s" % cpus, default=cpus, action="store", type="int")
+    parser.add_option("-d", "--delete", dest="delete", help="Clear all caches. Next time you render your world, it will have to start completely over again. This is probably not a good idea for large worlds. Use this if you change texture packs and want to re-render everything.", action="store_true")
 
     options, args = parser.parse_args()
 
@@ -34,6 +35,9 @@ def main():
         parser.error("Where do you want to save the tiles?")
     destdir = args[1]
 
+    if options.delete:
+        return delete_all(worlddir, destdir)
+
     # First generate the world's chunk images
     w = world.WorldRenderer(worlddir)
     w.go(options.procs)
@@ -42,5 +46,25 @@ def main():
     q = quadtree.QuadtreeGen(w, destdir)
     q.go(options.procs)
 
+def delete_all(worlddir, tiledir):
+    # First delete all images in the world dir
+    imgre = r"img\.[^.]+\.[^.]+\.nocave\.\w+\.png$"
+    matcher = re.compile(imgre)
+
+    for dirpath, dirnames, filenames in os.walk(worlddir):
+        for f in filenames:
+            if matcher.match(f):
+                filepath = os.path.join(dirpath, f)
+                print "Deleting {0}".format(filepath)
+                os.unlink(filepath)
+
+    # Now delete all /hash/ files in the tile dir.
+    for dirpath, dirnames, filenames in os.walk(tiledir):
+        for f in filenames:
+            if f.endswith(".hash"):
+                filepath = os.path.join(dirpath, f)
+                print "Deleting {0}".format(filepath)
+                os.unlink(filepath)
+
 if __name__ == "__main__":
     main()