diff --git a/spec/rng_spec.lua b/spec/rng_spec.lua
index b60f015..ae8d048 100644
--- a/spec/rng_spec.lua
+++ b/spec/rng_spec.lua
@@ -1,3 +1,100 @@
-pending("implement rng tests", function()
-  -- TODO: implement
+describe("rng's", function()
+
+  describe("math_random()", function()
+
+    it("generates 1 byte", function()
+      local rng = require("uuid").rng.math_random()
+
+      assert.is.string(rng(1))
+      assert.is.equal(1, #rng(1))
+    end)
+
+
+    it("generates 256 bytes", function()
+      local rng = require("uuid").rng.math_random()
+
+      assert.is.string(rng(256))
+      assert.is.equal(256, #rng(256))
+    end)
+
+  end)
+
+
+
+  describe("luasystem()", function()
+
+    it("picks up system.random", function()
+      package.loaded.system = {
+        random = function(n)
+          return string.char(0):rep(n)
+        end
+      }
+
+      local rng = require("uuid").rng.luasystem()
+
+      assert.are.equal(package.loaded.system.random, rng)
+    end)
+
+    it("errors if not available", function()
+      local old_require = _G.require
+      finally(function()
+        _G.require = old_require
+      end)
+      _G.require = function(name)
+        if name ~= "system" then
+          return old_require(name)
+        end
+        return error("not found")
+      end
+
+      local rng, err = require("uuid").rng.luasystem()
+      assert.is.falsy(rng)
+      assert.is.equal("luasystem not available", err)
+    end)
+
+    it("errors if too old", function()
+      package.loaded.system = {} -- not having 'random' function
+
+      local rng, err = require("uuid").rng.luasystem()
+      assert.is.falsy(rng)
+      assert.is.equal("luasystem version is too old, minimum version required is 0.3", err)
+    end)
+
+  end)
+
+
+
+  describe("urandom()", function()
+
+    it("generates 1 byte", function()
+      local rng = require("uuid").rng.urandom()
+
+      assert.is.string(rng(1))
+      assert.is.equal(1, #rng(1))
+    end)
+
+
+    it("generates 256 bytes", function()
+      local rng = require("uuid").rng.urandom()
+
+      assert.is.string(rng(256))
+      assert.is.equal(256, #rng(256))
+    end)
+
+    it("fails on Windows", function()
+      local old_package_config = package.config
+      finally(function()
+        package.config = old_package_config -- luacheck: ignore
+      end)
+      -- make it think this is Windows
+      package.config = "\\" .. package.config:sub(2,-1) -- luacheck: ignore
+
+      local rng, err = require("uuid").rng.urandom()
+      assert.is.falsy(rng)
+      assert.is.equal("/dev/urandom is not available on Windows", err)
+    end)
+
+  end)
+
+
 end)