diff --git a/project.clj b/project.clj
index 6a01432..92a7fed 100644
--- a/project.clj
+++ b/project.clj
@@ -3,7 +3,8 @@
   :url "https://gitlab.com/KKlochko/cipher-analytical-machine"
   :license {:name "LGPL"
             :url "http://www.gnu.org/licenses/lgpl-3.0.txt"}
-  :dependencies [[org.clojure/clojure "1.11.1"]]
+  :dependencies [[org.clojure/clojure "1.11.1"]
+                 [org.clojure/tools.cli "1.0.219"]]
   :main ^:skip-aot cipher-analytical-machine.core
   :target-path "target/%s"
   :profiles {:uberjar {:aot :all
diff --git a/src/cipher_analytical_machine/cli.clj b/src/cipher_analytical_machine/cli.clj
new file mode 100644
index 0000000..f0c3b9d
--- /dev/null
+++ b/src/cipher_analytical_machine/cli.clj
@@ -0,0 +1,82 @@
+(ns cipher-analytical-machine.cli
+  (:require
+   [cipher-analytical-machine.caesar :as caesar]
+   [clojure.string :as cs]
+   [clojure.tools.cli :refer [parse-opts]])
+  (:gen-class))
+
+(def cipher-options
+  #{"Caesar"})
+
+(def cli-options
+  [["-m" "--message MESSAGE" "The message will be encrypted or decrypted."]
+   ["-k" "--key KEY" "The key will be used to encrypt or decrypt."]
+   ["-c" "--cipher CIPHER" "The cipher will be used to encrypt or decrypt a message."
+    :default "Caesar"]
+   ["-e" "--encrypt" "Encrypt the message."]
+   ["-d" "--decrypt" "Decrypt the message."]
+   ["-h" "--help"]])
+
+(defn usage [options-summary]
+  (->> ["This is cipher-analytical-machine. It can help you to learn how ciphers works."
+        ""
+        "Usage: cipher-analytical-machine [options]"
+        ""
+        "Options:"
+        options-summary
+        ""]
+       (cs/join \newline)))
+
+(defn error-msg [errors]
+  (str "The following errors occurred while parsing your command:\n\n"
+       (cs/join \newline errors)))
+
+(defn map-has-keys?
+  "Return true if all keys in the map."
+  [map keys]
+  (every? #(contains? map %) keys))
+
+(defn validate-args
+  "Validate command line arguments. Either return a map indicating the program
+  should exit (with an error message, and optional ok status), or a map
+  indicating the action the program should take and the options provided."
+  [args]
+  (let [{:keys [options arguments errors summary]} (parse-opts args cli-options)]
+    (cond
+      (:help options)
+      {:exit-message (usage summary) :ok? true}
+
+      errors
+      {:exit-message (error-msg errors)}
+
+      (map-has-keys? options [:encrypt, :decrypt])
+      {:exit-message (error-msg ["You can't use enctypt and decrypt mode at the same time!!!"])}
+
+      (and (map-has-keys? options [:cipher, :key])
+           (contains? options :message)
+           (or (contains? options :encrypt)
+               (contains? options :decrypt)))
+      {:options options :arguments arguments}
+
+      :else
+      {:exit-message (usage summary)})))
+
+(defn exit
+  [exit-message status]
+  (println exit-message)
+  (System/exit (if status 0 1)))
+
+(defn actions
+  [options arguments]
+  (println (str options arguments))
+  (let [message (:message options)
+        key (Integer/parseInt (:key options))
+        symbols "abc"]
+    (println
+     (cond
+       (contains? options :encrypt)
+       (caesar/encrypt-message message key symbols)
+
+       (contains? options :decrypt)
+       (caesar/decrypt-message message key symbols)))))
+
diff --git a/src/cipher_analytical_machine/core.clj b/src/cipher_analytical_machine/core.clj
index 00a2e4e..3c19ef0 100644
--- a/src/cipher_analytical_machine/core.clj
+++ b/src/cipher_analytical_machine/core.clj
@@ -1,7 +1,12 @@
 (ns cipher-analytical-machine.core
+  (:require
+   [cipher-analytical-machine.cli :as cli])
   (:gen-class))
 
 (defn -main
-  "I don't do a whole lot ... yet."
   [& args]
-  (println "Hello, World!"))
+  (let [{:keys [options arguments exit-message ok?]} (cli/validate-args args)]
+    (if exit-message
+      (cli/exit exit-message ok?)
+      (cli/actions options arguments))))
+