diff --git a/ui/medashare/tags.py b/ui/medashare/tags.py
index acafb93..031cd5a 100644
--- a/ui/medashare/tags.py
+++ b/ui/medashare/tags.py
@@ -1,8 +1,10 @@
 import collections
 import itertools
+import math
 import pathlib
 import shutil
 import tempfile
+import time
 import unittest
 from .utils import _asn1coder
 
@@ -23,6 +25,10 @@ class TagCache:
 
 	tags is the initial starting state of the cache.
 
+	half_life is the half life of an tag measured in seconds.
+	Each time a tag is added, it counts as one more, but old
+	values are decayed per the specified half life.
+
 	Current use is to create a starter object as such:
 	cache = TagCache(), and modify count as needed:
 	cache.count = 10
@@ -35,10 +41,15 @@ class TagCache:
 		self._cache = collections.OrderedDict((x, None) for x in tags)
 		self._count = count
 		self._modified = False
-		if 'limit' in kwargs:
-			self._limit = kwargs['limit']
-		else:
-			self._limit = self._count
+
+		# λ = ln(2) / t1/2
+		hl = kwargs.pop('half_life', None)
+		self._lambda = None if not hl else math.log(2) / hl
+		self._limit = kwargs.pop('limit', self._count)
+
+		if kwargs:
+			raise TypeError('unknown kwarg(s): %s' %
+			    ', '.join(kwargs.keys()))
 
 	def _limit_count(self):
 		while len(self._cache) > self._limit:
@@ -83,20 +94,41 @@ class TagCache:
 
 		self._modified = True
 
-		try:
-			del self._cache[tag]
-		except KeyError:
-			pass
+		oldtime, oldval = self._cache.pop(tag, (0, 0))
 
-		self._cache[tag] = None
+		t = time.time()
+
+		# N(t) = N0 e^(-λt)
+		if self._lambda:
+			v = (t, oldval * math.exp(-self._lambda * (t -
+			    oldtime)) + 1)
+		else:
+			v = 0, 0
+
+		self._cache[tag] = v
 
 		self._limit_count()
 
+	def _update_values(self):
+		'''Update the values to match the current time.'''
+
+		t = time.time()
+
+		for k, (oldtime, oldval) in self._cache.items():
+			v = (t, oldval * math.exp(-self._lambda * (t -
+			    oldtime)) + 1)
+			self._cache[k] = v
+
 	def tags(self):
 		'''Returns the sorted list of tags in the cache.'''
 
-		return sorted(itertools.islice(self._cache.keys(),
-		    len(self._cache) - self._count, len(self._cache)))
+		if self._lambda:
+			self._update_values()
+			return sorted(sorted(self._cache.keys(), key=lambda x:
+			    self._cache[x][1], reverse=True)[:self._count])
+		else:
+			return sorted(itertools.islice(self._cache.keys(),
+			    len(self._cache) - self._count, len(self._cache)))
 
 	def __repr__(self):
 		return 'TagCache(tags=%s, count=%d, limit=%d)' % \
@@ -143,10 +175,28 @@ class _TestTagCache(unittest.TestCase):
 		shutil.rmtree(self.basetempdir)
 		self.tempdir = None
 
+	def test_extra_kwargs(self):
+		with self.assertRaises(TypeError):
+			TagCache(randomkwargs=True)
+
 	@unittest.mock.patch('time.time', side_effect=lambda cnt=
-	    itertools.count(): cnt.next() * 1.)
+	    itertools.count(): next(cnt) * 1.)
 	def test_halflife(self, tt):
-		pass
+		tc = TagCache(count=2, limit=10, half_life=10)
+
+		# that when it is added twice
+		tc.add(('foo', 'foo'))
+		tc.add(('foo', 'foo'))
+
+		# and two others are added
+		tc.add(('bar', 'bar'))
+		tc.add(('baz', 'baz'))
+
+		# it will have preference
+		self.assertEqual(tc.tags(), [ ('baz', 'baz'), ('foo', 'foo'), ])
+
+		# XXX - deal with limit better, that is, drop the small value,
+		# not the last
 
 	def test_limit(self):
 		tc = TagCache(count=2, limit=3)