diff --git a/acme/acme/client.py b/acme/acme/client.py
index 0e9319f9c..082dcb5cd 100644
--- a/acme/acme/client.py
+++ b/acme/acme/client.py
@@ -37,6 +37,7 @@ class Client(object):  # pylint: disable=too-many-instance-attributes
     :ivar key: `.JWK` (private)
     :ivar alg: `.JWASignature`
     :ivar bool verify_ssl: Verify SSL certificates?
+    :ivar str ua: User agent string to send to the server.
     :ivar .ClientNetwork net: Client network. Useful for testing. If not
         supplied, it will be initialized using `key`, `alg` and
         `verify_ssl`.
@@ -45,7 +46,7 @@ class Client(object):  # pylint: disable=too-many-instance-attributes
     DER_CONTENT_TYPE = 'application/pkix-cert'
 
     def __init__(self, directory, key, alg=jose.RS256, verify_ssl=True,
-                 net=None):
+                 net=None, ua="acme-python"):
         """Initialize.
 
         :param directory: Directory Resource (`.messages.Directory`) or
@@ -53,7 +54,7 @@ class Client(object):  # pylint: disable=too-many-instance-attributes
 
         """
         self.key = key
-        self.net = ClientNetwork(key, alg, verify_ssl) if net is None else net
+        self.net = ClientNetwork(key, alg, verify_ssl, user_agent) if net is None else net
 
         if isinstance(directory, six.string_types):
             self.directory = messages.Directory.from_json(
diff --git a/letsencrypt/cli.py b/letsencrypt/cli.py
index c37533115..6f4facc3e 100644
--- a/letsencrypt/cli.py
+++ b/letsencrypt/cli.py
@@ -169,6 +169,7 @@ def _determine_account(args, config):
 
 
 def _init_le_client(args, config, authenticator, installer):
+    config.deterimine_user_agent(authenticator, installer)
     if authenticator is not None:
         # if authenticator was given, then we will need account...
         acc, acme = _determine_account(args, config)
@@ -521,6 +522,7 @@ def revoke(args, config, unused_plugins):  # TODO: coop with renewal config
         logger.debug("Revoking %s using Account Key", args.cert_path[0])
         acc, _ = _determine_account(args, config)
         # pylint: disable=protected-access
+        config.determine_user_agent(None, None)
         acme = client._acme_from_config_key(config, acc.key)
     acme.revoke(jose.ComparableX509(crypto_util.pyopenssl_load_certificate(
         args.cert_path[1])[0]))
diff --git a/letsencrypt/client.py b/letsencrypt/client.py
index 8a0ad6af4..0b4297791 100644
--- a/letsencrypt/client.py
+++ b/letsencrypt/client.py
@@ -32,7 +32,8 @@ logger = logging.getLogger(__name__)
 def _acme_from_config_key(config, key):
     # TODO: Allow for other alg types besides RS256
     return acme_client.Client(directory=config.server, key=key,
-                              verify_ssl=(not config.no_verify_ssl))
+                              verify_ssl=(not config.no_verify_ssl),
+                              ua=config.user_agent)
 
 
 def register(config, account_storage, tos_cb=None):
diff --git a/letsencrypt/configuration.py b/letsencrypt/configuration.py
index e70171675..a3fd33d69 100644
--- a/letsencrypt/configuration.py
+++ b/letsencrypt/configuration.py
@@ -3,6 +3,7 @@ import os
 import urlparse
 import re
 
+import letsencrypt
 import zope.interface
 
 from acme import challenges
@@ -82,6 +83,19 @@ class NamespaceConfig(object):
         else:
             return challenges.HTTP01Response.PORT
 
+    def determine_user_agent(self, authenticator, installer):
+        # The user agent string isn't knowable until the authenticator and
+        # installer have been chosen.
+
+        if self.user_agent is None:
+            ua = "LetsEncryptPythonClient/{0} ({1}) Authenticator/{2} Installer/{3}"
+            ua = ua.format(letsencrypt.__version__, le_util.get_os_info(),
+                           authenticator.name if authenticator else "none",
+                           installer.name if installer else "none")
+            self.user_agent=ua
+        else:
+            assert isinstance(self.user_agent, str), "User Agent not a string?"
+
 
 class RenewerConfiguration(object):
     """Configuration wrapper for renewer."""
diff --git a/letsencrypt/le_util.py b/letsencrypt/le_util.py
index 413cec43d..4a780e36b 100644
--- a/letsencrypt/le_util.py
+++ b/letsencrypt/le_util.py
@@ -3,6 +3,7 @@ import collections
 import errno
 import logging
 import os
+import platform
 import re
 import subprocess
 import stat
@@ -226,7 +227,7 @@ def get_os_info():
         os_ver = os_ver.parititon("-")[0]
         os_ver = os_ver.parititon(".")[0]
     elif platform.win32_ver()[1]:
-        os_ver = win32_ver()[1]
+        os_ver = platform.win32_ver()[1]
     else:
         # Cases known to fall here: Cygwin python
         os_ver = ''