diff --git a/lib/live_book/evaluator.ex b/lib/live_book/evaluator.ex
index c07b90d47..9b4a1a95f 100644
--- a/lib/live_book/evaluator.ex
+++ b/lib/live_book/evaluator.ex
@@ -46,23 +46,19 @@ defmodule LiveBook.Evaluator do
   end
 
   @doc """
-  Synchronously parses and evaluates the given code.
+  Asynchronously parses and evaluates the given code.
 
   Any exceptions are captured, in which case this method returns an error.
 
   The evaluator stores the resulting binding and environment under `ref`.
   Any subsequent calls may specify `prev_ref` pointing to a previous evaluation,
   in which case the corresponding binding and environment are used during evaluation.
+
+  Evaluation response is sent to the process identified by `send_to` as `{:evaluation_response, response}`.
   """
-  @spec evaluate_code(t(), String.t(), ref(), ref()) :: evaluation_response()
-  def evaluate_code(evaluator, code, ref, prev_ref \\ :initial) when ref != :initial do
-    response = GenServer.call(evaluator, {:evaluate_code, code, ref, prev_ref}, :infinity)
-
-    if response == :invalid_prev_ref do
-      raise ArgumentError, message: "invalid reference to previous evaluation: #{prev_ref}"
-    end
-
-    response
+  @spec evaluate_code(t(), pid(), String.t(), ref(), ref()) :: :ok
+  def evaluate_code(evaluator, send_to, code, ref, prev_ref \\ :initial) when ref != :initial do
+    GenServer.cast(evaluator, {:evaluate_code, send_to, code, ref, prev_ref})
   end
 
   @doc """
@@ -108,29 +104,26 @@ defmodule LiveBook.Evaluator do
   end
 
   @impl true
-  def handle_call({:evaluate_code, code, ref, prev_ref}, {from, _}, state) do
-    case Map.fetch(state.contexts, prev_ref) do
-      :error ->
-        {:reply, :invalid_prev_ref, state}
+  def handle_cast({:evaluate_code, send_to, code, ref, prev_ref}, state) do
+    Evaluator.IOProxy.configure(state.io_proxy, send_to, ref)
 
-      {:ok, context} ->
-        Evaluator.IOProxy.configure(state.io_proxy, from, ref)
+    context = Map.get(state.contexts, prev_ref, state.contexts.initial)
 
-        case eval(code, context.binding, context.env) do
-          {:ok, result, binding, env} ->
-            result_context = %{binding: binding, env: env}
-            new_contexts = Map.put(state.contexts, ref, result_context)
-            new_state = %{state | contexts: new_contexts}
+    case eval(code, context.binding, context.env) do
+      {:ok, result, binding, env} ->
+        result_context = %{binding: binding, env: env}
+        new_contexts = Map.put(state.contexts, ref, result_context)
+        new_state = %{state | contexts: new_contexts}
 
-            {:reply, {:ok, result}, new_state}
+        send(send_to, {:evaluator_response, {:ok, result}})
+        {:noreply, new_state}
 
-          {:error, kind, error, stacktrace} ->
-            {:reply, {:error, kind, error, stacktrace}, state}
-        end
+      {:error, kind, error, stacktrace} ->
+        send(send_to, {:evaluator_response, {:error, kind, error, stacktrace}})
+        {:noreply, state}
     end
   end
 
-  @impl true
   def handle_cast({:forget_evaluation, ref}, state) do
     new_state = %{state | contexts: Map.delete(state.contexts, ref)}
     {:noreply, new_state}
diff --git a/lib/live_book/session.ex b/lib/live_book/session.ex
index 0dea6ad7a..7b608c011 100644
--- a/lib/live_book/session.ex
+++ b/lib/live_book/session.ex
@@ -233,7 +233,6 @@ defmodule LiveBook.Session do
     {:ok, section} = Notebook.fetch_cell_section(notebook, cell_id)
     {state, evaluator} = get_section_evaluator(state, section.id)
     %{source: source} = cell
-    session_pid = self()
 
     prev_ref =
       case Notebook.parent_cells(notebook, cell_id) do
@@ -241,10 +240,7 @@ defmodule LiveBook.Session do
         [] -> :initial
       end
 
-    spawn(fn ->
-      response = Evaluator.evaluate_code(evaluator, source, cell_id, prev_ref)
-      send(session_pid, {:evaluator_response, cell_id, response})
-    end)
+    Evaluator.evaluate_code(evaluator, self(), source, cell_id, prev_ref)
 
     state
   end
diff --git a/test/live_book/evaluator_test.exs b/test/live_book/evaluator_test.exs
index bcd4ad712..456cf62f8 100644
--- a/test/live_book/evaluator_test.exs
+++ b/test/live_book/evaluator_test.exs
@@ -16,44 +16,45 @@ defmodule LiveBook.EvaluatorTest do
       x + y
       """
 
-      result = Evaluator.evaluate_code(evaluator, code, 1)
+      Evaluator.evaluate_code(evaluator, self(), code, 1)
 
-      assert result == {:ok, 3}
+      assert_receive {:evaluator_response, {:ok, 3}}
     end
 
     test "given no prev_ref does not see previous evaluation context", %{evaluator: evaluator} do
-      Evaluator.evaluate_code(evaluator, "x = 1", :code_1)
+      Evaluator.evaluate_code(evaluator, self(), "x = 1", :code_1)
 
-      result = Evaluator.evaluate_code(evaluator, "x", :code_2)
+      Evaluator.evaluate_code(evaluator, self(), "x", :code_2)
 
-      assert {:error, _kind, %CompileError{description: "undefined function x/0"}, _stacktrace} =
-               result
+      assert_receive {:evaluator_response,
+                      {:error, _kind, %CompileError{description: "undefined function x/0"},
+                       _stacktrace}}
     end
 
     test "given prev_ref sees previous evaluation context", %{evaluator: evaluator} do
-      Evaluator.evaluate_code(evaluator, "x = 1", :code_1)
+      Evaluator.evaluate_code(evaluator, self(), "x = 1", :code_1)
 
-      result = Evaluator.evaluate_code(evaluator, "x", :code_2, :code_1)
+      Evaluator.evaluate_code(evaluator, self(), "x", :code_2, :code_1)
 
-      assert result == {:ok, 1}
+      assert_receive {:evaluator_response, {:ok, 1}}
     end
 
-    test "given invalid prev_ref raises an error", %{evaluator: evaluator} do
-      assert_raise ArgumentError, fn ->
-        Evaluator.evaluate_code(evaluator, ":ok", :code_1, :code_nonexistent)
-      end
+    test "given invalid prev_ref just uses default context", %{evaluator: evaluator} do
+      Evaluator.evaluate_code(evaluator, self(), ":hey", :code_1, :code_nonexistent)
+
+      assert_receive {:evaluator_response, {:ok, :hey}}
     end
 
     test "captures standard output and sends it to the caller", %{evaluator: evaluator} do
-      Evaluator.evaluate_code(evaluator, ~s{IO.puts("hey")}, :code_1)
+      Evaluator.evaluate_code(evaluator, self(), ~s{IO.puts("hey")}, :code_1)
 
-      assert_received {:evaluator_stdout, :code_1, "hey\n"}
+      assert_receive {:evaluator_stdout, :code_1, "hey\n"}
     end
 
     test "using standard input results in an immediate error", %{evaluator: evaluator} do
-      result = Evaluator.evaluate_code(evaluator, ~s{IO.gets("> ")}, :code_1)
+      Evaluator.evaluate_code(evaluator, self(), ~s{IO.gets("> ")}, :code_1)
 
-      assert result == {:ok, {:error, :enotsup}}
+      assert_receive {:evaluator_response, {:ok, {:error, :enotsup}}}
     end
 
     test "returns error along with its kind and stacktrace", %{evaluator: evaluator} do
@@ -61,9 +62,10 @@ defmodule LiveBook.EvaluatorTest do
       List.first(%{})
       """
 
-      result = Evaluator.evaluate_code(evaluator, code, :code_1)
+      Evaluator.evaluate_code(evaluator, self(), code, :code_1)
 
-      assert {:error, :error, %FunctionClauseError{}, [{List, :first, 1, _location}]} = result
+      assert_receive {:evaluator_response,
+                      {:error, :error, %FunctionClauseError{}, [{List, :first, 1, _location}]}}
     end
 
     test "in case of an error returns only the relevant part of stacktrace", %{
@@ -87,25 +89,28 @@ defmodule LiveBook.EvaluatorTest do
       Cat.meow()
       """
 
-      result = Evaluator.evaluate_code(evaluator, code, :code_1)
+      Evaluator.evaluate_code(evaluator, self(), code, :code_1)
 
       expected_stacktrace = [
         {Math, :bad_math, 0, [file: 'nofile', line: 3]},
         {Cat, :meow, 0, [file: 'nofile', line: 10]}
       ]
 
-      assert {:error, _kind, _error, ^expected_stacktrace} = result
+      assert_receive {:evaluator_response, {:error, _kind, _error, ^expected_stacktrace}}
     end
   end
 
   describe "forget_evaluation/2" do
     test "invalidates the given reference", %{evaluator: evaluator} do
-      Evaluator.evaluate_code(evaluator, "x = 1", :code_1)
-      Evaluator.forget_evaluation(evaluator, :code_1)
+      Evaluator.evaluate_code(evaluator, self(), "x = 1", :code_1)
+      assert_receive {:evaluator_response, _}
 
-      assert_raise ArgumentError, fn ->
-        Evaluator.evaluate_code(evaluator, ":ok", :code_2, :code_1)
-      end
+      Evaluator.forget_evaluation(evaluator, :code_1)
+      Evaluator.evaluate_code(evaluator, self(), "x", :code_2, :code_1)
+
+      assert_receive {:evaluator_response,
+                      {:error, _kind, %CompileError{description: "undefined function x/0"},
+                       _stacktrace}}
     end
   end
 end