diff --git a/.gitignore b/.gitignore
index cb856ac..4212989 100644
--- a/.gitignore
+++ b/.gitignore
@@ -10,4 +10,7 @@ pom.xml.asc
 .hgignore
 .hg/
 /resources/*
-*~
\ No newline at end of file
+*~
+*.ods
+
+*.csv
diff --git a/project.clj b/project.clj
index bbac0a0..3ef2533 100644
--- a/project.clj
+++ b/project.clj
@@ -5,6 +5,7 @@
             :url "https://www.gnu.org/licenses/old-licenses/gpl-2.0.en.html"}
   :dependencies [[org.clojure/clojure "1.8.0"]
                  [org.clojure/data.json "0.2.6"]
+                 [org.clojure/tools.cli "0.3.7"]
                  [clj-http "3.9.1"]
                  [clj-time "0.14.4"]]
   :aot [mastodonusers.core]
diff --git a/src/mastodonusers/core.clj b/src/mastodonusers/core.clj
index 2d58255..9ecbe01 100644
--- a/src/mastodonusers/core.clj
+++ b/src/mastodonusers/core.clj
@@ -1,25 +1,65 @@
 (ns mastodonusers.core
   (:require [clojure.data.json :as json]
             [clojure.pprint :refer :all]
+            [clojure.string :as s]
+            [clojure.tools.cli :refer [parse-opts]]
             [clojure.walk :refer [keywordize-keys]]
             [clj-http.client :as h]
             [clj-time.core :as t]
             [clj-time.format :as f])
   (:gen-class))
 
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;;;;
+;;;; mastodonusers.core
+;;;;
+;;;; This program is free software; you can redistribute it and/or
+;;;; modify it under the terms of the GNU General Public License
+;;;; as published by the Free Software Foundation; either version 2
+;;;; of the License, or (at your option) any later version.
+;;;;
+;;;; This program is distributed in the hope that it will be useful,
+;;;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;;;; GNU General Public License for more details.
+;;;;
+;;;; You should have received a copy of the GNU General Public License
+;;;; along with this program; if not, write to the Free Software
+;;;; Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301,
+;;;; USA.
+;;;;
+;;;; Copyright (C) 2018 Simon Brooke
+;;;;
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
-;; (def application-id "431276798")
-;; (def application-name "mastodonusers")
+(def cli-options
+  [["-h" "--help" "Show this message"
+    :default false]
+   ["-o" "--csv" "output data as comma-separated values"]
+   ["-s" "--bearer-secret [SECRET]" "The secret which identifies us to instances.social"]
+   ["-v" "--verbosity [LEVEL]" nil "Verbosity level - integer value required"
+    :parse-fn #(Integer/parseInt %)
+    :default 0]
+   ])
+
+
+(defn usage
+  "Show a usage message. `options` should be options as
+  parsed by [clojure.tools.cli](https://github.com/clojure/tools.cli)"
+  [options]
+  (println (:summary options)))
 
 
 (defn fetch-data
-  "Fetch current data from instances.social with our key"
-  []
+  "Fetch current data from instances.social with this `secret` key"
+  [secret]
   (h/get
    "https://instances.social/api/1.0/instances/list"
    {:headers {:authorization (str
                               "Bearer "
-                              (slurp "resources/secret.txt"))}
+                              (or
+                               secret
+                               (slurp "resources/secret.txt")))}
     :query-params {:count 0}
     :accept :json}))
 
@@ -61,14 +101,68 @@
         c)))))
 
 
-(defn -main [& args]
+(defn ->csv
+  "Return the contents of this EDN `file`, assumed to contain a map with
+  date/time keys and count values, in CSV format."
+  [file]
+  (let [data (read-string (slurp file))]
+    (s/join
+     "\n"
+     (map
+      #(str % ", " (data %))
+      (sort
+       #(compare
+         (f/parse
+          ;; (f/formatters :basic-date-time)
+          %1)
+         (f/parse
+          ;; (f/formatters :basic-date-time)
+          %2))
+       (keys data))))))
+
+
+(defn process
+  "Process these parsed `options`."
+  [options]
+  (let [c (count-users
+           (fetch-data
+            (-> options :options :bearer-secret)))
+        args (:arguments options)]
+    (if-not
+      (empty? args)
+      (update-file (first args) c))
+    (if-not
+      (zero? (-> options :options :verbosity))
+      (println "Total #fediverse user accounts: " c))
+    (if
+      (and
+       (not (empty? args))
+       (-> options :options :csv))
+      (println (->csv (first args))))
+    c))
+
+
+(defn -main
   "Print a count of the current number of user accounts in the fediverse.
   If an argument is present, it is presumed to be the pathname of
   an EDN formatted file containing a map mapping times to counts, which will
   be updated with the current count."
-  (let [c (count-users (fetch-data))]
-    (if-not (empty? args) (update-file (first args) c))
-    (println "Total #fediverse user accounts: " c)
-    c))
+  [& args]
+  (let [options (parse-opts args cli-options)]
+    (cond
+     (empty? args)
+     (usage options)
+     (seq (:errors options))
+     (do
+       (doall
+        (map
+         println
+         (:errors options)))
+       (usage options))
+     (-> options :options :help)
+     (usage options)
+     true
+     (process options))))
+