diff --git a/2023/09/day09.clj b/2023/09/day09.clj
new file mode 100644
index 0000000..0c8bae6
--- /dev/null
+++ b/2023/09/day09.clj
@@ -0,0 +1,59 @@
+(ns day09
+  (:require [clojure.string :as s]
+            [clojure.edn :as edn]))
+
+(defn parse-line [line]
+  (->> (s/split line #" ")
+       (map edn/read-string)))
+
+(defn parse-input [input]
+  (->> (s/split-lines input)
+       (map parse-line)))
+
+(defn differences [steps]
+  (vec (map - (drop 1 steps) steps)))
+
+(defn all-zeroes? [steps]
+  (every? zero? steps))
+
+(defn expand-line [steps]
+  (let [diffs (differences steps)]
+    (if (all-zeroes? diffs)
+      (last steps)
+      (let [added (expand-line diffs)]
+        (+ added (last steps))))))
+
+(defn part1 [steps]
+  (reduce + (map expand-line steps)))
+
+(defn part2 [steps]
+  (part1 (map reverse steps)))
+
+(comment
+  (= [1 2 3]
+     (parse-line "1 2 3"))
+  (= [[1 2 3] [4 5 6]]
+     (parse-input "1 2 3\n4 5 6"))
+  (differences [1 2 3])
+  (= [3 3 3 3 3]
+     (differences [0 3 6 9 12 15]))
+  (= [0 0 0 0]
+     (differences [3 3 3 3 3]))
+  (= 18
+     (expand-line [0 3 6 9 12 15]))
+  (= 28
+     (expand-line [1 3 6 10 15 21]))
+  (= 68
+     (expand-line [10 13 16 21 30 45]))
+
+  (let [example (parse-input (slurp "example"))]
+    (and (= 114 (part1 example))
+         (= 2 (part2 example)))))
+
+;; Invoke with clj -X day09/main
+;; Or with bb --init day09.clj -e '(day09/main)'
+(defn -main [& args]
+  (let [input (parse-input (slurp "input"))]
+    (do
+      (println (str "Part 1: " (part1 input)))
+      (println (str "Part 2: " (part2 input))))))
diff --git a/2023/09/deps.edn b/2023/09/deps.edn
new file mode 100644
index 0000000..a8e645f
--- /dev/null
+++ b/2023/09/deps.edn
@@ -0,0 +1,5 @@
+
+{:paths ["."]
+ :aliases
+ {:nREPL {:extra-deps
+          {nrepl/nrepl {:mvn/version "1.0.0"}}}}}
diff --git a/2023/09/flake.lock b/2023/09/flake.lock
new file mode 100644
index 0000000..ec3d1fa
--- /dev/null
+++ b/2023/09/flake.lock
@@ -0,0 +1,60 @@
+{
+  "nodes": {
+    "flake-utils": {
+      "inputs": {
+        "systems": "systems"
+      },
+      "locked": {
+        "lastModified": 1701680307,
+        "narHash": "sha256-kAuep2h5ajznlPMD9rnQyffWG8EM/C73lejGofXvdM8=",
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "rev": "4022d587cbbfd70fe950c1e2083a02621806a725",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "type": "github"
+      }
+    },
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1701680448,
+        "narHash": "sha256-jyBmBxAIOgSxuViuffo4ah8+NRHfY/9ByozjiLk/rf4=",
+        "owner": "nixos",
+        "repo": "nixpkgs",
+        "rev": "6f3d9c7288802bd463f1419c1354a2d8b5ad1d9a",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nixos",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "flake-utils": "flake-utils",
+        "nixpkgs": "nixpkgs"
+      }
+    },
+    "systems": {
+      "locked": {
+        "lastModified": 1681028828,
+        "narHash": "sha256-Vy1rq5AaRuLzOxct8nz4T6wlgyUR7zLU309k9mBC768=",
+        "owner": "nix-systems",
+        "repo": "default",
+        "rev": "da67096a3b9bf56a91d16901293e51ba5b49a27e",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-systems",
+        "repo": "default",
+        "type": "github"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}
diff --git a/2023/09/flake.nix b/2023/09/flake.nix
new file mode 100644
index 0000000..f886e39
--- /dev/null
+++ b/2023/09/flake.nix
@@ -0,0 +1,20 @@
+{
+  description = "A flake for pulling in dependencies.";
+
+  inputs = {
+
+    nixpkgs.url = "github:nixos/nixpkgs";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = { self, nixpkgs, flake-utils, ... }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let
+        pkgs = import nixpkgs { inherit system; };
+      in rec {
+        devShells.default = with pkgs; mkShell {
+          buildInputs = [ clojure ];
+        };
+      }
+    );
+}