diff --git a/config/test.exs b/config/test.exs
index fc6404a..4e2df64 100644
--- a/config/test.exs
+++ b/config/test.exs
@@ -21,5 +21,8 @@ config :phoenix, :plug_init_mode, :runtime
 
 config :chronoscope, Chronoscope.NTS,
   behaviour: Chronoscope.NTS.BehaviourMock,
-  datetime: Chronoscope.NTS.DateTimeMock,
-  ssl: Chronoscope.NTS.SSLMock
+  date_time: Chronoscope.NTS.DateTimeMock,
+  ssl: Chronoscope.NTS.SSLMock,
+  registry: Chronoscope.NTS.RegistryMock,
+  dynamic_supervisor: Chronoscope.NTS.DynamicSupervisorMock,
+  gen_server: Chronoscope.NTS.GenServerMock
diff --git a/lib/chronoscope/nts.ex b/lib/chronoscope/nts.ex
index 6d211d6..ccd8ed8 100644
--- a/lib/chronoscope/nts.ex
+++ b/lib/chronoscope/nts.ex
@@ -9,22 +9,26 @@ defmodule Chronoscope.NTS do
 
   alias Chronoscope.NTS
 
+  @registry Application.compile_env(:chronoscope, Chronoscope.NTS)[:registry] || Registry
+  @genserver Application.compile_env(:chronoscope, Chronoscope.NTS)[:gen_server] || GenServer
+  @dynamic_supervisor Application.compile_env(:chronoscope, Chronoscope.NTS)[:dynamic_supervisor] || DynamicSupervisor
+
   def healthy?() do
     true
   end
 
   def list() do
     NTS.DynamicSupervisor
-    |> DynamicSupervisor.which_children()
-    |> Enum.map(fn {_, pid, _, _} -> GenServer.call(pid, :list) end)
+    |> @dynamic_supervisor.which_children()
+    |> Enum.map(fn {_, pid, _, _} -> @genserver.call(pid, :list) end)
   end
 
   def remove(host, port) do
     name = client_name(%{host: host, port: port})
 
-    case Registry.lookup(NTS.Registry, name) do
-      [{pid, _}] -> GenServer.call(pid, :terminate)
-      [] -> {:error, :notfound}
+    case @registry.lookup(NTS.Registry, name) do
+      [{pid, _}] -> {:ok, @genserver.call(pid, :terminate)}
+      [] -> {:error, :not_found}
     end
   end
 
@@ -32,13 +36,13 @@ defmodule Chronoscope.NTS do
   def key_establishment(host, port) do
     %{host: host, port: port}
     |> client_pid()
-    |> GenServer.call(:key_establishment)
+    |> @genserver.call(:key_establishment)
   end
 
   defp client_pid(server) do
     name = client_name(server)
 
-    case Registry.lookup(NTS.Registry, name) do
+    case @registry.lookup(NTS.Registry, name) do
       [{pid, _}] -> pid
       [] -> start_client(server, name)
     end
@@ -50,7 +54,7 @@ defmodule Chronoscope.NTS do
 
   defp start_client(server, name) do
     NTS.DynamicSupervisor
-    |> DynamicSupervisor.start_child({NTS.Client, server: server, name: {:via, Registry, {NTS.Registry, name}}})
+    |> @dynamic_supervisor.start_child({NTS.Client, server: server, name: {:via, @registry, {NTS.Registry, name}}})
     |> then(fn {:ok, pid} -> pid end)
   end
 end
diff --git a/lib/chronoscope/nts/certificate.ex b/lib/chronoscope/nts/certificate.ex
index 31000f2..e9f4e76 100644
--- a/lib/chronoscope/nts/certificate.ex
+++ b/lib/chronoscope/nts/certificate.ex
@@ -1,4 +1,6 @@
 defmodule Chronoscope.NTS.Certificate do
+  @date_time Application.compile_env(:chronoscope, Chronoscope.NTS)[:date_time] || DateTime
+
   def expiration_date(certificate) do
     {:Validity, _, {:utcTime, expiration}} =
       certificate
@@ -20,7 +22,7 @@ defmodule Chronoscope.NTS.Certificate do
 
   defp short_year_to_full_year(short_year) do
     {century, current_year} =
-      datetime().utc_now().year
+      @date_time.utc_now().year
       |> to_string()
       |> String.split_at(-2)
 
@@ -32,8 +34,4 @@ defmodule Chronoscope.NTS.Certificate do
       |> then(&"#{&1 + 1}#{short_year}")
     end
   end
-
-  defp datetime() do
-    Application.get_env(:chronoscope, Chronoscope.NTS)[:datetime] || DateTime
-  end
 end
diff --git a/lib/chronoscope/nts/client.ex b/lib/chronoscope/nts/client.ex
index d1fff9c..c887c32 100644
--- a/lib/chronoscope/nts/client.ex
+++ b/lib/chronoscope/nts/client.ex
@@ -5,6 +5,7 @@ defmodule Chronoscope.NTS.Client do
   alias Chronoscope.NTS.KeyEstablishmentClient
 
   @interval_in_seconds 30
+  @date_time Application.compile_env(:chronoscope, Chronoscope.NTS)[:date_time] || DateTime
 
   def start_link(server: server, name: name) do
     GenServer.start_link(__MODULE__, server, name: name)
@@ -70,10 +71,6 @@ defmodule Chronoscope.NTS.Client do
   end
 
   defp utc_now() do
-    datetime().utc_now()
-  end
-
-  defp datetime() do
-    Application.get_env(:chronoscope, Chronoscope.NTS)[:datetime] || DateTime
+    @date_time.utc_now()
   end
 end
diff --git a/lib/chronoscope/nts/key_establishment_client.ex b/lib/chronoscope/nts/key_establishment_client.ex
index 4612b78..a72e2c8 100644
--- a/lib/chronoscope/nts/key_establishment_client.ex
+++ b/lib/chronoscope/nts/key_establishment_client.ex
@@ -6,6 +6,7 @@ defmodule Chronoscope.NTS.KeyEstablishmentClient do
   alias Chronoscope.NTS.KeyEstablishmentResponse
 
   @timeout_in_milliseconds 3000
+  @ssl Application.compile_env(:chronoscope, Chronoscope.NTS)[:ssl] || :ssl
 
   def key_establishment(%{host: host, port: port}) do
     case ssl_connect(host, port) do
@@ -21,7 +22,7 @@ defmodule Chronoscope.NTS.KeyEstablishmentClient do
   defp ssl_connect(host, port) do
     host
     |> String.to_charlist()
-    |> ssl().connect(port, tls_options(host), @timeout_in_milliseconds)
+    |> @ssl.connect(port, tls_options(host), @timeout_in_milliseconds)
   end
 
   defp tls_options(host) do
@@ -31,21 +32,21 @@ defmodule Chronoscope.NTS.KeyEstablishmentClient do
   end
 
   defp perform_key_establishment(socket) do
-    :ok = ssl().send(socket, KeyEstablishmentRequest.create())
-    {:ok, peercert} = ssl().peercert(socket)
+    :ok = @ssl.send(socket, KeyEstablishmentRequest.create())
+    {:ok, peercert} = @ssl.peercert(socket)
 
     receive do
       {:ssl, _socket, response} ->
-        ssl().close(socket)
+        @ssl.close(socket)
         parse_response(response, peercert)
 
       msg ->
-        ssl().close(socket)
+        @ssl.close(socket)
         Logger.error("received unexpected message: #{inspect(msg)}")
         {:error, :no_response}
     after
       @timeout_in_milliseconds ->
-        ssl().close(socket)
+        @ssl.close(socket)
         Logger.error("timed out waiting for response")
         {:error, :timeout}
     end
@@ -67,8 +68,4 @@ defmodule Chronoscope.NTS.KeyEstablishmentClient do
         {:error, String.trim(error)}
     end
   end
-
-  defp ssl() do
-    Application.get_env(:chronoscope, Chronoscope.NTS)[:ssl] || :ssl
-  end
 end
diff --git a/lib/chronoscope_web/controllers/api/v1/nts/key_establishment_controller.ex b/lib/chronoscope_web/controllers/api/v1/nts/key_establishment_controller.ex
index 3a1f4e0..ffe5c94 100644
--- a/lib/chronoscope_web/controllers/api/v1/nts/key_establishment_controller.ex
+++ b/lib/chronoscope_web/controllers/api/v1/nts/key_establishment_controller.ex
@@ -7,6 +7,7 @@ defmodule ChronoscopeWeb.API.V1.NTS.KeyEstablishmentController do
 
   @default_port 4460
   @max_host_length 255
+  @nts Application.compile_env(:chronoscope, NTS)[:behaviour] || NTS
 
   def get(conn, %{"host" => host, "port" => port}) do
     try do
@@ -41,7 +42,7 @@ defmodule ChronoscopeWeb.API.V1.NTS.KeyEstablishmentController do
   defp key_establishment_response(host, port) do
     host
     |> String.slice(0, @max_host_length)
-    |> nts_behaviour().key_establishment(port)
+    |> @nts.key_establishment(port)
   end
 
   defp format_response(response) do
@@ -55,8 +56,4 @@ defmodule ChronoscopeWeb.API.V1.NTS.KeyEstablishmentController do
     |> put_status(:bad_request)
     |> json(%{error: message})
   end
-
-  defp nts_behaviour() do
-    Application.get_env(:chronoscope, NTS)[:behaviour] || NTS
-  end
 end
diff --git a/test/chronoscope/nts_test.exs b/test/chronoscope/nts_test.exs
index c8fbefc..08b3b25 100644
--- a/test/chronoscope/nts_test.exs
+++ b/test/chronoscope/nts_test.exs
@@ -1,7 +1,14 @@
 defmodule Chronoscope.NTSTest do
   use Chronoscope.Case
 
+  alias Chronoscope.NTS.DynamicSupervisorMock
+  alias Chronoscope.NTS.GenServerMock
+  alias Chronoscope.NTS.RegistryMock
+
   import Chronoscope.NTS
+  import Mox
+
+  setup :verify_on_exit!
 
   describe "Chronoscope.NTS.healthy?()" do
     test "is healthy" do
@@ -11,13 +18,30 @@ defmodule Chronoscope.NTSTest do
 
   describe "Chronoscope.NTS.list()" do
     test "shows empty client list" do
+      DynamicSupervisorMock
+      |> expect(:which_children, fn _ -> [] end)
+
       assert list() == []
     end
+
+    test "lists all children" do
+      DynamicSupervisorMock
+      |> expect(:which_children, fn _ -> [{1, 2, 3, 4}, {5, 6, 7, 8}] end)
+
+      GenServerMock
+      |> expect(:call, fn 2, :list -> :one end)
+      |> expect(:call, fn 6, :list -> :two end)
+
+      assert list() == [:one, :two]
+    end
   end
 
   describe "Chronoscope.NTS.remove()" do
     test "does nothing if the client doesn't exist" do
-      assert remove("localhost", 1111) == {:error, :notfound}
+      RegistryMock
+      |> expect(:lookup, fn _, _ -> [] end)
+
+      assert remove("localhost", 1111) == {:error, :not_found}
     end
   end
 end
diff --git a/test/support/behaviours.ex b/test/support/behaviours.ex
new file mode 100644
index 0000000..b5003f6
--- /dev/null
+++ b/test/support/behaviours.ex
@@ -0,0 +1,23 @@
+defmodule Chronoscope.DateTime.Behaviour do
+  @callback utc_now :: DateTime.t()
+end
+
+defmodule Chronoscope.SSL.Behaviour do
+  @callback connect(any(), any(), any(), any()) :: {:ok, any()} | {:error, any()}
+  @callback send(any(), any()) :: :ok | {:error, any()}
+  @callback peercert(any()) :: {:ok, any()} | {:error, any()}
+  @callback close(any()) :: :ok | {:error, any()}
+end
+
+defmodule Chronoscope.Registry.Behaviour do
+  @callback lookup(atom(), any()) :: [{pid(), any()}]
+end
+
+defmodule Chronoscope.DynamicSupervisor.Behaviour do
+  @callback start_child(Supervisor.supervisor(), any()) :: any()
+  @callback which_children(Supervisor.supervisor()) :: [any()]
+end
+
+defmodule Chronoscope.GenServer.Behaviour do
+  @callback call(pid(), any()) :: any()
+end
diff --git a/test/support/mocks.ex b/test/support/mocks.ex
index 37ee853..53a182e 100644
--- a/test/support/mocks.ex
+++ b/test/support/mocks.ex
@@ -1,19 +1,6 @@
-defmodule Chronoscope.DateTime.Behaviour do
-  @callback utc_now :: DateTime.t()
-end
-
-defmodule Chronoscope.DateTime.Stub do
-  @behaviour Chronoscope.DateTime.Behaviour
-  def utc_now(), do: DateTime.utc_now()
-end
-
-defmodule Chronoscope.SSL.Behaviour do
-  @callback connect(any(), any(), any(), any()) :: {:ok, any()} | {:error, any()}
-  @callback send(any(), any()) :: :ok | {:error, any()}
-  @callback peercert(any()) :: {:ok, any()} | {:error, any()}
-  @callback close(any()) :: :ok | {:error, any()}
-end
-
 Mox.defmock(Chronoscope.NTS.BehaviourMock, for: Chronoscope.NTS.Behaviour)
 Mox.defmock(Chronoscope.NTS.DateTimeMock, for: Chronoscope.DateTime.Behaviour)
 Mox.defmock(Chronoscope.NTS.SSLMock, for: Chronoscope.SSL.Behaviour)
+Mox.defmock(Chronoscope.NTS.RegistryMock, for: Chronoscope.Registry.Behaviour)
+Mox.defmock(Chronoscope.NTS.DynamicSupervisorMock, for: Chronoscope.DynamicSupervisor.Behaviour)
+Mox.defmock(Chronoscope.NTS.GenServerMock, for: Chronoscope.GenServer.Behaviour)
diff --git a/test/support/stubs.ex b/test/support/stubs.ex
new file mode 100644
index 0000000..92ec93e
--- /dev/null
+++ b/test/support/stubs.ex
@@ -0,0 +1,4 @@
+defmodule Chronoscope.DateTime.Stub do
+  @behaviour Chronoscope.DateTime.Behaviour
+  def utc_now(), do: DateTime.utc_now()
+end