diff --git a/app/assets/javascripts/editor/editor.js.erb b/app/assets/javascripts/editor/editor.js.erb
index f05914e6..970a4696 100644
--- a/app/assets/javascripts/editor/editor.js.erb
+++ b/app/assets/javascripts/editor/editor.js.erb
@@ -773,6 +773,8 @@ var CodeOceanEditor = {
             this.showContainerDepletedMessage();
         } else if (output.status === 'out_of_memory') {
             this.showOutOfMemoryMessage();
+        } else if (output.status === 'runner_in_use') {
+            this.showRunnerInUseMessage();
         }
     },
 
@@ -832,6 +834,13 @@ var CodeOceanEditor = {
         });
     },
 
+    showRunnerInUseMessage: function () {
+        $.flash.warning({
+            icon: ['fa-solid', 'fa-triangle-exclamation'],
+            text: I18n.t('exercises.editor.runner_in_use')
+        });
+    },
+
     showTimeoutMessage: function () {
         $.flash.info({
             icon: ['fa-regular', 'fa-clock'],
diff --git a/app/assets/javascripts/editor/evaluation.js b/app/assets/javascripts/editor/evaluation.js
index 317a3a86..c493f98d 100644
--- a/app/assets/javascripts/editor/evaluation.js
+++ b/app/assets/javascripts/editor/evaluation.js
@@ -109,6 +109,11 @@ CodeOceanEditorEvaluation = {
         })) {
             this.showOutOfMemoryMessage();
         }
+        if (_.some(response, function (result) {
+            return result.status === 'runner_in_use';
+        })) {
+            this.showRunnerInUseMessage();
+        }
         if (_.some(response, function (result) {
             return result.status === 'container_depleted';
         })) {
diff --git a/app/controllers/execution_environments_controller.rb b/app/controllers/execution_environments_controller.rb
index 39c09c43..640e47de 100644
--- a/app/controllers/execution_environments_controller.rb
+++ b/app/controllers/execution_environments_controller.rb
@@ -33,7 +33,7 @@ class ExecutionEnvironmentsController < ApplicationController
   def execute_command
     runner = Runner.for(current_user, @execution_environment)
     @privileged_execution = ActiveModel::Type::Boolean.new.cast(params[:sudo]) || @execution_environment.privileged_execution
-    output = runner.execute_command(params[:command], privileged_execution: @privileged_execution, raise_exception: false)
+    output = runner.execute_command(params[:command], privileged_execution: @privileged_execution, raise_exception: false, exclusive: false)
     render json: output.except(:messages)
   end
 
@@ -41,11 +41,11 @@ class ExecutionEnvironmentsController < ApplicationController
     runner = Runner.for(current_user, @execution_environment)
     @privileged_execution = ActiveModel::Type::Boolean.new.cast(params[:sudo]) || @execution_environment.privileged_execution
     begin
-      files = runner.retrieve_files(path: params[:path], recursive: false, privileged_execution: @privileged_execution)
+      files = runner.retrieve_files(path: params[:path], recursive: false, privileged_execution: @privileged_execution, exclusive: false)
       downloadable_files, additional_directories = convert_files_json_to_files files
       js_tree = FileTree.new(downloadable_files, additional_directories, force_closed: true).to_js_tree
       render json: js_tree[:core][:data]
-    rescue Runner::Error::RunnerNotFound, Runner::Error::WorkspaceError
+    rescue Runner::Error::RunnerNotFound, Runner::Error::WorkspaceError, Runner::Error::RunnerInUse
       render json: []
     end
   end
diff --git a/app/controllers/live_streams_controller.rb b/app/controllers/live_streams_controller.rb
index 47f5b313..c51a6075 100644
--- a/app/controllers/live_streams_controller.rb
+++ b/app/controllers/live_streams_controller.rb
@@ -26,15 +26,15 @@ class LiveStreamsController < ApplicationController
     runner = Runner.for(current_user, @execution_environment)
     fallback_location = shell_execution_environment_path(@execution_environment)
     privileged = params[:sudo] || @execution_environment.privileged_execution?
-    send_runner_file(runner, desired_file, fallback_location, privileged:)
+    send_runner_file(runner, desired_file, fallback_location, privileged:, exclusive: false)
   end
 
   private
 
-  def send_runner_file(runner, desired_file, redirect_fallback = root_path, privileged: false)
+  def send_runner_file(runner, desired_file, redirect_fallback = root_path, privileged: false, exclusive: true)
     filename = File.basename(desired_file)
     send_stream(filename:, type: 'application/octet-stream', disposition: 'attachment') do |stream|
-      runner.download_file(desired_file, privileged_execution: privileged) do |chunk, overall_size, _content_type|
+      runner.download_file(desired_file, privileged_execution: privileged, exclusive:) do |chunk, overall_size, _content_type|
         unless response.committed?
           # Disable Rack::ETag, which would otherwise cause the response to be cached
           # See https://github.com/rack/rack/issues/1619#issuecomment-848460528
diff --git a/app/controllers/remote_evaluation_controller.rb b/app/controllers/remote_evaluation_controller.rb
index 1c4b4b6b..caa63f66 100644
--- a/app/controllers/remote_evaluation_controller.rb
+++ b/app/controllers/remote_evaluation_controller.rb
@@ -72,6 +72,9 @@ class RemoteEvaluationController < ApplicationController
       # TODO: check token expired?
       {message: 'No exercise found for this validation_token! Please keep out!', status: 401}
     end
+  rescue Runner::Error::RunnerInUse => e
+    Rails.logger.debug { "Scoring a submission failed because the runner was already in use: #{e.message}" }
+    {message: I18n.t('exercises.editor.runner_in_use'), status: 409}
   rescue Runner::Error => e
     Rails.logger.debug { "Runner error while scoring submission #{@submission.id}: #{e.message}" }
     {message: I18n.t('exercises.editor.depleted'), status: 503}
diff --git a/app/controllers/request_for_comments_controller.rb b/app/controllers/request_for_comments_controller.rb
index 634fa048..b92de462 100644
--- a/app/controllers/request_for_comments_controller.rb
+++ b/app/controllers/request_for_comments_controller.rb
@@ -167,11 +167,20 @@ class RequestForCommentsController < ApplicationController
 
     respond_to do |format|
       if @request_for_comment.save
-        # execute the tests here and wait until they finished.
-        # As the same runner is used for the score and test run, no parallelization is possible
-        # A run is triggered from the frontend and does not need to be handled here.
-        @request_for_comment.submission.calculate_score(current_user)
-        format.json { render :show, status: :created, location: @request_for_comment }
+        begin
+          # execute the tests here and wait until they finished.
+          # As the same runner is used for the score and test run, no parallelization is possible
+          # A run is triggered from the frontend and does not need to be handled here.
+          @request_for_comment.submission.calculate_score(current_user)
+        rescue Runner::Error::RunnerInUse => e
+          Rails.logger.debug { "Scoring a submission failed because the runner was already in use: #{e.message}" }
+          format.json { render json: {error: t('exercises.editor.runner_in_use'), status: :runner_in_use}, status: :conflict }
+        rescue Runner::Error => e
+          Rails.logger.debug { "Runner error while requesting comments: #{e.message}" }
+          format.json { render json: {danger: t('exercises.editor.depleted'), status: :container_depleted}, status: :service_unavailable }
+        else
+          format.json { render :show, status: :created, location: @request_for_comment }
+        end
       else
         format.html { render :new }
         format.json { render json: @request_for_comment.errors, status: :unprocessable_entity }
diff --git a/app/controllers/submissions_controller.rb b/app/controllers/submissions_controller.rb
index b60eb36a..9283063b 100644
--- a/app/controllers/submissions_controller.rb
+++ b/app/controllers/submissions_controller.rb
@@ -234,6 +234,12 @@ class SubmissionsController < ApplicationController
     @testrun[:exit_code] ||= 137
     @testrun[:output] = "out_of_memory: #{@testrun[:output]}"
     extract_durations(e)
+  rescue Runner::Error::RunnerInUse => e
+    send_and_store client_socket, {cmd: :status, status: :runner_in_use}
+    Rails.logger.debug { "Running a submission failed because the runner was already in use: #{e.message}" }
+    @testrun[:status] ||= :runner_in_use
+    @testrun[:output] = "runner_in_use: #{@testrun[:output]}"
+    extract_durations(e)
   rescue Runner::Error => e
     # Regardless of the specific error cause, we send a `container_depleted` status to the client.
     send_and_store client_socket, {cmd: :status, status: :container_depleted}
@@ -266,6 +272,14 @@ class SubmissionsController < ApplicationController
     client_socket&.send_data(JSON.dump(@submission.calculate_score(current_user)))
     # To enable hints when scoring a submission, uncomment the next line:
     # send_hints(client_socket, StructuredError.where(submission: @submission))
+  rescue Runner::Error::RunnerInUse => e
+    extract_durations(e)
+    send_and_store client_socket, {cmd: :status, status: :runner_in_use}
+    Rails.logger.debug { "Scoring a submission failed because the runner was already in use: #{e.message}" }
+    @testrun[:passed] = false
+    @testrun[:status] ||= :runner_in_use
+    @testrun[:output] = "runner_in_use: #{@testrun[:output]}"
+    save_testrun_output 'assess'
   rescue Runner::Error => e
     extract_durations(e)
     send_and_store client_socket, {cmd: :status, status: :container_depleted}
@@ -301,10 +315,17 @@ class SubmissionsController < ApplicationController
 
     # The score is stored separately, we can forward it to the client immediately
     client_socket&.send_data(JSON.dump(@submission.test(@file, current_user)))
+  rescue Runner::Error::RunnerInUse => e
+    extract_durations(e)
+    send_and_store client_socket, {cmd: :status, status: :runner_in_use}
+    Rails.logger.debug { "Scoring a submission failed because the runner was already in use: #{e.message}" }
+    @testrun[:passed] = false
+    @testrun[:status] ||= :runner_in_use
+    @testrun[:output] = "runner_in_use: #{@testrun[:output]}"
+    save_testrun_output 'assess'
   rescue Runner::Error => e
     extract_durations(e)
     send_and_store client_socket, {cmd: :status, status: :container_depleted}
-    kill_client_socket(client_socket)
     Rails.logger.debug { "Runner error while testing submission #{@submission.id}: #{e.message}" }
     Sentry.capture_exception(e)
     @testrun[:passed] = false
diff --git a/app/errors/runner/error.rb b/app/errors/runner/error.rb
index cc88de17..0084902c 100644
--- a/app/errors/runner/error.rb
+++ b/app/errors/runner/error.rb
@@ -16,6 +16,8 @@ class Runner
 
     class Unauthorized < Error; end
 
+    class RunnerInUse < Error; end
+
     class RunnerNotFound < Error; end
 
     class FaradayError < Error; end
diff --git a/app/models/execution_environment.rb b/app/models/execution_environment.rb
index 7e5fa9ac..9bb3eab7 100644
--- a/app/models/execution_environment.rb
+++ b/app/models/execution_environment.rb
@@ -92,7 +92,7 @@ class ExecutionEnvironment < ApplicationRecord
     retries = 0
     begin
       runner = Runner.for(author, self)
-      output = runner.execute_command(VALIDATION_COMMAND)
+      output = runner.execute_command(VALIDATION_COMMAND, exclusive: false)
       errors.add(:docker_image, "error: #{output[:stderr]}") if output[:stderr].present?
     rescue Runner::Error => e
       # In case of an Runner::Error, we retry multiple times before giving up.
diff --git a/app/models/runner.rb b/app/models/runner.rb
index 293a6c18..dfcd16d1 100644
--- a/app/models/runner.rb
+++ b/app/models/runner.rb
@@ -44,18 +44,24 @@ class Runner < ApplicationRecord
   end
 
   def copy_files(files)
+    reserve!
     @strategy.copy_files(files)
   rescue Runner::Error::RunnerNotFound
     request_new_id
     save
     @strategy.copy_files(files)
+  ensure
+    release!
   end
 
-  def download_file(...)
-    @strategy.download_file(...)
+  def download_file(desired_file, privileged_execution:, exclusive: true)
+    reserve! if exclusive
+    @strategy.download_file(desired_file, privileged_execution:)
+    release! if exclusive
   end
 
-  def retrieve_files(raise_exception: true, **)
+  def retrieve_files(raise_exception: true, exclusive: true, **)
+    reserve! if exclusive
     try = 0
     begin
       if try.nonzero?
@@ -77,12 +83,14 @@ class Runner < ApplicationRecord
       # We forward the exception if requested
       raise e if raise_exception && defined?(e) && e.present?
 
-      # Otherwise, we return an hash with empty files
+      # Otherwise, we return an hash with empty files and release the runner
+      release! if exclusive
       {'files' => []}
     end
   end
 
-  def attach_to_execution(command, privileged_execution: false, &block)
+  def attach_to_execution(command, privileged_execution: false, exclusive: true, &block)
+    reserve! if exclusive
     Rails.logger.debug { "#{Time.zone.now.getutc.inspect}: Starting execution with Runner #{id} for #{contributor_type} #{contributor_id}." }
     starting_time = Time.zone.now
     begin
@@ -100,11 +108,12 @@ class Runner < ApplicationRecord
       e.execution_duration = Time.zone.now - starting_time
       raise
     end
+    release! if exclusive
     Rails.logger.debug { "#{Time.zone.now.getutc.inspect}: Stopped execution with Runner #{id} for #{contributor_type} #{contributor_id}." }
     Time.zone.now - starting_time # execution duration
   end
 
-  def execute_command(command, privileged_execution: false, raise_exception: true)
+  def execute_command(command, privileged_execution: false, raise_exception: true, exclusive: true)
     output = {
       stdout: +'',
       stderr: +'',
@@ -119,7 +128,7 @@ class Runner < ApplicationRecord
         save
       end
 
-      execution_time = attach_to_execution(command, privileged_execution:) do |socket, starting_time|
+      execution_time = attach_to_execution(command, privileged_execution:, exclusive:) do |socket, starting_time|
         socket.on :stderr do |data|
           output[:stderr] << data
           output[:messages].push({cmd: :write, stream: :stderr, log: data, timestamp: Time.zone.now - starting_time})
@@ -139,6 +148,9 @@ class Runner < ApplicationRecord
     rescue Runner::Error::OutOfMemory => e
       Rails.logger.debug { "Running command `#{command}` caused an out of memory error: #{e.message}" }
       output.merge!(status: :out_of_memory, container_execution_time: e.execution_duration)
+    rescue Runner::Error::RunnerInUse => e
+      Rails.logger.debug { "Running command `#{command}` failed because the runner was already in use: #{e.message}" }
+      output.merge!(status: :runner_in_use, container_execution_time: e.execution_duration)
     rescue Runner::Error::RunnerNotFound => e
       Rails.logger.debug { "Running command `#{command}` failed for the first time: #{e.message}" }
       try += 1
@@ -166,6 +178,27 @@ class Runner < ApplicationRecord
 
   def destroy_at_management
     @strategy.destroy_at_management
+    update!(runner_id: nil, reserved_until: nil)
+  end
+
+  def reserve!
+    with_lock do
+      if reserved_until.present? && reserved_until > Time.zone.now
+        @error = Runner::Error::RunnerInUse.new("The desired Runner #{id} is already in use until #{reserved_until.iso8601}.")
+        raise @error
+      else
+        update!(reserved_until: Time.zone.now + execution_environment.permitted_execution_time.seconds)
+        @error = nil
+      end
+    end
+  end
+
+  def release!
+    return if @error.present?
+
+    with_lock do
+      update!(reserved_until: nil)
+    end
   end
 
   private
diff --git a/app/models/testrun.rb b/app/models/testrun.rb
index ed680461..9131790c 100644
--- a/app/models/testrun.rb
+++ b/app/models/testrun.rb
@@ -14,6 +14,7 @@ class Testrun < ApplicationRecord
     timeout: 3,
     out_of_memory: 4,
     terminated_by_client: 5,
+    runner_in_use: 6,
   }, _default: :ok, _prefix: true
 
   validates :exit_code, numericality: {only_integer: true, min: 0, max: 255}, allow_nil: true
diff --git a/config/locales/de.yml b/config/locales/de.yml
index dcc63768..8c2a08d9 100644
--- a/config/locales/de.yml
+++ b/config/locales/de.yml
@@ -398,6 +398,7 @@ de:
       network: 'Während Ihr Code läuft, ist Port %{port} unter folgender Adresse erreichbar: <a href="%{address}" target="_blank" rel="noopener">%{address}</a>.'
       render: Anzeigen
       run: Ausführen
+      runner_in_use: Sie führen momentan Code aus. Bitte stoppen Sie die vorherige Ausführung oder warten Sie einen Moment, bevor Sie fortfahren.
       run_failure: Ihr Code konnte nicht auf der Plattform ausgeführt werden.
       run_success: Ihr Code wurde auf der Plattform ausgeführt.
       requestComments: Kommentare erbitten
diff --git a/config/locales/en.yml b/config/locales/en.yml
index a42d0ae7..6ccb59c5 100644
--- a/config/locales/en.yml
+++ b/config/locales/en.yml
@@ -398,6 +398,7 @@ en:
       network: 'While your code is running, port %{port} is accessible using the following address: <a href="%{address}" target="_blank" rel="noopener">%{address}</a>.'
       render: Render
       run: Run
+      runner_in_use: You are currently running code. Please stop the previous execution, or wait a moment before proceeding.
       run_failure: Your code could not be run.
       run_success: Your code was run on our servers.
       requestComments: Request Comments
diff --git a/db/migrate/20231029172331_add_reserved_until_to_runners.rb b/db/migrate/20231029172331_add_reserved_until_to_runners.rb
new file mode 100644
index 00000000..b4045eb6
--- /dev/null
+++ b/db/migrate/20231029172331_add_reserved_until_to_runners.rb
@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class AddReservedUntilToRunners < ActiveRecord::Migration[7.1]
+  def change
+    add_column :runners, :reserved_until, :datetime, null: true, default: nil
+  end
+end
diff --git a/db/schema.rb b/db/schema.rb
index 29b6abbb..560eb378 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[7.1].define(version: 2023_10_13_194635) do
+ActiveRecord::Schema[7.1].define(version: 2023_10_29_172331) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_trgm"
   enable_extension "pgcrypto"
@@ -483,6 +483,7 @@ ActiveRecord::Schema[7.1].define(version: 2023_10_13_194635) do
     t.bigint "contributor_id"
     t.datetime "created_at", null: false
     t.datetime "updated_at", null: false
+    t.datetime "reserved_until"
     t.index ["contributor_type", "contributor_id"], name: "index_runners_on_user"
     t.index ["execution_environment_id"], name: "index_runners_on_execution_environment_id"
   end
diff --git a/spec/models/execution_environment_spec.rb b/spec/models/execution_environment_spec.rb
index bbaeddab..fc03843e 100644
--- a/spec/models/execution_environment_spec.rb
+++ b/spec/models/execution_environment_spec.rb
@@ -175,7 +175,7 @@ RSpec.describe ExecutionEnvironment do
     it 'executes the validation command' do
       allow(runner).to receive(:execute_command).and_return({})
       working_docker_image?
-      expect(runner).to have_received(:execute_command).with(ExecutionEnvironment::VALIDATION_COMMAND)
+      expect(runner).to have_received(:execute_command).with(ExecutionEnvironment::VALIDATION_COMMAND, exclusive: false)
     end
 
     context 'when the command produces an error' do