diff --git a/app/controllers/stages_controller.rb b/app/controllers/stages_controller.rb
new file mode 100644
index 0000000..cdc5f1b
--- /dev/null
+++ b/app/controllers/stages_controller.rb
@@ -0,0 +1,62 @@
+# frozen_string_literal: true
+
+class StagesController < ApplicationController
+  before_action :set_stage, only: %i[show update]
+  before_action :authenticate_user!, only: %i[update]
+  before_action -> { require_owner! @stage.owner }, only: %i[update]
+
+  # GET /stages/1
+  def show
+    render json: @stage, include: '**'
+  end
+
+  # PUT /stages/1
+  def update
+    if stage_params[:state] == 'finished'
+      unless @stage.state == 'in_progress'
+        render json: { error: 'Only running group stages can be finished' }, status: :unprocessable_entity
+        return
+      end
+
+      Stage.transaction do
+        if @stage.update(stage_params)
+          handle_group_stage_end
+
+          render json: @stage
+        else
+          render json: @stage.errors, status: :unprocessable_entity
+          raise ActiveRecord::Rollback
+        end
+      end
+    else
+      render json: {
+        error: 'The state attribute may only be changed to finished'
+      }, status: :unprocessable_entity
+    end
+  end
+
+  private
+
+  def handle_group_stage_end
+    unless @stage.over?
+      render json: {
+        error: 'Group Stage still has some matches that are not over yet. Finish them to generate playoffs'
+      }, status: :unprocessable_entity
+      raise ActiveRecord::Rollback
+    end
+
+    return if AddPlayoffsToTournamentAndSave.call(tournament: @stage.tournament,
+                                                  teams: GroupStageService.get_advancing_teams(@stage)).success?
+
+    render json: { error: 'Generating group stage failed' }, status: :unprocessable_entity
+    raise ActiveRecord::Rollback
+  end
+
+  def set_stage
+    @stage = Stage.find(params[:id])
+  end
+
+  def stage_params
+    params.slice(:state).permit!
+  end
+end
diff --git a/app/controllers/tournaments_controller.rb b/app/controllers/tournaments_controller.rb
index 78bcfee..5d225b7 100644
--- a/app/controllers/tournaments_controller.rb
+++ b/app/controllers/tournaments_controller.rb
@@ -49,7 +49,7 @@ class TournamentsController < ApplicationController
       # associate provided teams with tournament
       tournament.teams = teams
       # add playoff stage to tournament
-      result = AddPlayoffsToTournamentAndSave.call(tournament: tournament)
+      result = AddPlayoffsToTournamentAndSave.call(tournament: tournament, teams: tournament.teams)
     end
     # validate tournament
     unless tournament.valid?
diff --git a/app/interactors/add_playoffs_to_tournament.rb b/app/interactors/add_playoffs_to_tournament.rb
index ce530cb..9abecdf 100644
--- a/app/interactors/add_playoffs_to_tournament.rb
+++ b/app/interactors/add_playoffs_to_tournament.rb
@@ -6,7 +6,7 @@ class AddPlayoffsToTournament
   def call
     tournament = context.tournament
     context.fail! if tournament.stages.size > 1
-    if (playoff_stages = PlayoffStageService.generate_playoff_stages_from_tournament(tournament))
+    if (playoff_stages = PlayoffStageService.generate_playoff_stages(context.teams, context.randomize_matches))
       if tournament.stages.empty?
         tournament.stages = playoff_stages
       else
diff --git a/app/models/group_score.rb b/app/models/group_score.rb
index d7686d3..077b081 100644
--- a/app/models/group_score.rb
+++ b/app/models/group_score.rb
@@ -3,4 +3,8 @@
 class GroupScore < ApplicationRecord
   belongs_to :team
   belongs_to :group
+
+  def difference_in_points
+    scored_points - received_points
+  end
 end
diff --git a/app/models/stage.rb b/app/models/stage.rb
index f29d661..8e4267f 100644
--- a/app/models/stage.rb
+++ b/app/models/stage.rb
@@ -18,4 +18,14 @@ class Stage < ApplicationRecord
       []
     end
   end
+
+  def over?
+    return matches.find { |m| m.state != 'finished' }.nil? unless matches.size.zero?
+
+    unless groups.size.zero? && groups.map(&:matches).flatten.size.zero?
+      return groups.map(&:matches).flatten.find { |m| m.state != 'finished' }.nil?
+    end
+
+    false
+  end
 end
diff --git a/app/services/group_stage_service.rb b/app/services/group_stage_service.rb
index 5e92a1d..6490980 100644
--- a/app/services/group_stage_service.rb
+++ b/app/services/group_stage_service.rb
@@ -55,5 +55,38 @@ class GroupStageService
       end
       changed_group_scores
     end
+
+    # Returns a list of the teams in the group sorted by their group_points, difference_in_points, scored_points
+    #
+    # @param group Group the group to get the teams from
+    # @return [Array] of teams
+    def teams_sorted_by_group_scores(group)
+      group.teams.sort do |a, b|
+        group_score_a = group.group_scores.find_by(team: a)
+        group_score_b = group.group_scores.find_by(team: b)
+
+        [group_score_b.group_points,
+         group_score_b.difference_in_points,
+         group_score_b.scored_points] <=>
+          [group_score_a.group_points,
+           group_score_a.difference_in_points,
+           group_score_a.scored_points]
+      end
+    end
+
+    # Returns all teams advancing to playoff stage from given group stage
+    # They are ordered in such a way, that PlayoffStageService will correctly match the teams
+    #
+    # @param group_stage GroupStage the group stage to get all advancing teams from
+    # @return [Array] the teams advancing from that group stage
+    def get_advancing_teams(group_stage)
+      advancing_teams = []
+      group_winners = group_stage.groups.map(&method(:teams_sorted_by_group_scores))
+      (group_stage.tournament.instant_finalists_amount + group_stage.tournament.intermediate_round_participants_amount)
+        .times do |i|
+        advancing_teams << group_winners[i % group_stage.groups.size].shift
+      end
+      advancing_teams
+    end
   end
 end
diff --git a/app/services/playoff_stage_service.rb b/app/services/playoff_stage_service.rb
index 11b28b1..e12ae2c 100644
--- a/app/services/playoff_stage_service.rb
+++ b/app/services/playoff_stage_service.rb
@@ -6,11 +6,12 @@ class PlayoffStageService
     #
     # @param teams [Array] The teams to generate the playoff stages with
     # @return [Array] the generated playoff stages
-    def generate_playoff_stages(teams)
+    def generate_playoff_stages(teams, randomize_matches)
       playoffs = []
       stage_count = calculate_required_stage_count(teams.size)
       # initial_matches are the matches in the first stage; this is the only stage filled with teams from the start on
       initial_matches = MatchService.generate_matches(teams)
+      initial_matches = initial_matches.shuffle.each_with_index { |m, i| m.position = i } if randomize_matches
       initial_stage = Stage.new level: stage_count - 1, matches: initial_matches
       initial_stage.state = :intermediate_stage unless initial_stage.matches.find(&:single_team?).nil?
       playoffs << initial_stage
@@ -20,14 +21,6 @@ class PlayoffStageService
       playoffs
     end
 
-    # Generates the playoff stage given the tournament
-    #
-    # @param tournament [Tournament] The tournament to generate the playoff stages from
-    # @return [Array] the generated playoff stages
-    def generate_playoff_stages_from_tournament(tournament)
-      generate_playoff_stages tournament.teams
-    end
-
     # Generates given number of empty stages
     #
     # @param stage_count [Integer] number of stages to generate
diff --git a/config/routes.rb b/config/routes.rb
index d8f761a..61901bd 100644
--- a/config/routes.rb
+++ b/config/routes.rb
@@ -9,6 +9,7 @@ Rails.application.routes.draw do
   resources :matches, only: %i[show update] do
     resources :bets, only: %i[index create]
   end
+  resources :stages, only: %i[show update]
   resources :teams, only: %i[show update]
   resources :tournaments do
     resources :statistics, only: %i[index]
diff --git a/spec/controllers/stages_controller_spec.rb b/spec/controllers/stages_controller_spec.rb
new file mode 100644
index 0000000..a292400
--- /dev/null
+++ b/spec/controllers/stages_controller_spec.rb
@@ -0,0 +1,140 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe StagesController, type: :controller do
+  let(:stage) do
+    create(:playoff_stage)
+  end
+
+  describe 'GET #show' do
+    it 'returns a success response' do
+      get :show, params: { id: stage.to_param }
+      expect(response).to be_successful
+    end
+
+    it 'should return the correct stage' do
+      get :show, params: { id: stage.to_param }
+      body = deserialize_response response
+      expect(Stage.find_by(id: body[:id])).to eq(stage)
+      expect(body[:level]).to eq(stage.level)
+      expect(body[:state]).to eq(stage.state)
+    end
+  end
+
+  describe 'PUT #update' do
+    let(:finished) do
+      { state: 'finished' }
+    end
+
+    context 'group_stage with matches that are done' do
+      let(:running_group_stage) do
+        create(:group_stage, match_factory: :finished_group_match)
+      end
+
+      it 'doesn\'t have any other stages besides it before update' do
+        expect(running_group_stage.tournament.stages.size).to eq(1)
+      end
+
+      context 'as owner' do
+        before(:each) do
+          apply_authentication_headers_for running_group_stage.owner
+        end
+
+        before do
+          put :update, params: { id: running_group_stage.to_param }.merge(finished)
+          running_group_stage.reload
+        end
+
+        it 'succeeds' do
+          expect(response).to be_successful
+        end
+
+        it 'stops the stage' do
+          expect(running_group_stage.state).to eq('finished')
+        end
+
+        it 'adds new stages to the tournament' do
+          expect(running_group_stage.tournament.stages.size).to be > 1
+        end
+
+        it 'adds the right teams' do
+          expect(running_group_stage.tournament.stages.max_by(&:level).teams)
+            .to match_array(GroupStageService.get_advancing_teams(running_group_stage))
+        end
+      end
+
+      context 'as another user' do
+        before(:each) do
+          apply_authentication_headers_for create(:user)
+        end
+
+        it 'returns an error' do
+          put :update, params: { id: stage.to_param }.merge(finished)
+          expect(response).to have_http_status(:forbidden)
+        end
+      end
+    end
+
+    context 'trying to finish a group stage with unfinished matches' do
+      let(:group_stage) do
+        create(:group_stage)
+      end
+
+      before do
+        apply_authentication_headers_for group_stage.owner
+        put :update, params: { id: group_stage.to_param }.merge(finished)
+      end
+
+      it 'it returns unprocessable entity' do
+        expect(response).to have_http_status(:unprocessable_entity)
+      end
+
+      it 'returns the correct error' do
+        expect(deserialize_response(response)[:error])
+          .to eq('Group Stage still has some matches that are not over yet. Finish them to generate playoffs')
+      end
+    end
+
+    context 'already finished group stage' do
+      let(:finished_group_stage) do
+        group_stage = create(:group_stage, match_factory: :finished_group_match)
+        group_stage.finished!
+        group_stage.save!
+        group_stage
+      end
+
+      before do
+        apply_authentication_headers_for finished_group_stage.owner
+        put :update, params: { id: finished_group_stage.to_param }.merge(finished)
+      end
+
+      it 'it returns unprocessable entity' do
+        expect(response).to have_http_status(:unprocessable_entity)
+      end
+
+      it 'returns the correct error' do
+        expect(deserialize_response(response)[:error]).to eq('Only running group stages can be finished')
+      end
+    end
+
+    context 'trying to change the state to something other than :finished' do
+      let(:group_stage) do
+        create(:group_stage)
+      end
+
+      before do
+        apply_authentication_headers_for group_stage.owner
+        put :update, params: { id: group_stage.to_param }.merge(state: 'in_progress')
+      end
+
+      it 'it returns unprocessable entity' do
+        expect(response).to have_http_status(:unprocessable_entity)
+      end
+
+      it 'returns the correct error' do
+        expect(deserialize_response(response)[:error]).to eq('The state attribute may only be changed to finished')
+      end
+    end
+  end
+end
diff --git a/spec/factories/group_scores.rb b/spec/factories/group_scores.rb
index 558484a..0692ee6 100644
--- a/spec/factories/group_scores.rb
+++ b/spec/factories/group_scores.rb
@@ -4,5 +4,9 @@ FactoryBot.define do
   factory :group_score do
     team
     group
+
+    group_points { rand 5 }
+    scored_points { rand 5 }
+    received_points { rand 5 }
   end
 end
diff --git a/spec/factories/matches.rb b/spec/factories/matches.rb
index cd0543f..6b9facd 100644
--- a/spec/factories/matches.rb
+++ b/spec/factories/matches.rb
@@ -12,6 +12,9 @@ FactoryBot.define do
         match.match_scores = create_list(:match_score, evaluator.match_scores_count)
       end
       state { :in_progress }
+      factory :finished_playoff_match do
+        state { :finished }
+      end
     end
 
     factory :single_team_match do
@@ -53,6 +56,10 @@ FactoryBot.define do
       after(:create) do |match, evaluator|
         match.match_scores = create_list(:match_score, evaluator.match_scores_count)
       end
+
+      factory :finished_group_match do
+        state { :finished }
+      end
     end
 
     factory :undecided_group_match do
diff --git a/spec/interactors/add_playoffs_to_tournament_interactor_spec.rb b/spec/interactors/add_playoffs_to_tournament_interactor_spec.rb
index 0ee2cbc..e1cebfc 100644
--- a/spec/interactors/add_playoffs_to_tournament_interactor_spec.rb
+++ b/spec/interactors/add_playoffs_to_tournament_interactor_spec.rb
@@ -2,15 +2,15 @@
 
 RSpec.describe AddPlayoffsToTournament, type: :interactor do
   let(:group_stage_tournament_context) do
-    AddPlayoffsToTournament.call(tournament: @group_stage_tournament)
+    AddPlayoffsToTournament.call(tournament: @group_stage_tournament, teams: @group_stage_tournament.teams)
   end
 
   let(:playoff_stage_tournament_context) do
-    AddPlayoffsToTournament.call(tournament: @playoff_stage_tournament)
+    AddPlayoffsToTournament.call(tournament: @playoff_stage_tournament, teams: @playoff_stage_tournament.teams)
   end
 
   let(:full_tournament_context) do
-    AddPlayoffsToTournament.call(tournament: @full_tournament)
+    AddPlayoffsToTournament.call(tournament: @full_tournament, teams: @full_tournament.teams)
   end
 
   before do
@@ -23,7 +23,7 @@ RSpec.describe AddPlayoffsToTournament, type: :interactor do
   context 'PlayoffStageService mocked' do
     before do
       expect(class_double('PlayoffStageService').as_stubbed_const(transfer_nested_constants: true))
-        .to receive(:generate_playoff_stages_from_tournament)
+        .to receive(:generate_playoff_stages)
         .and_return(@stages)
     end
 
@@ -53,7 +53,7 @@ RSpec.describe AddPlayoffsToTournament, type: :interactor do
   context 'playoff generation fails' do
     before do
       expect(class_double('PlayoffStageService').as_stubbed_const(transfer_nested_constants: true))
-        .to receive(:generate_playoff_stages_from_tournament)
+        .to receive(:generate_playoff_stages)
         .and_return(nil)
     end
 
diff --git a/spec/models/stage_spec.rb b/spec/models/stage_spec.rb
index ac1dc72..9fdd105 100644
--- a/spec/models/stage_spec.rb
+++ b/spec/models/stage_spec.rb
@@ -39,5 +39,65 @@ RSpec.describe Stage, type: :model do
         expect(teams).to match_array(@teams)
       end
     end
+
+    context 'empty stage' do
+      it 'returns an empty Array' do
+        expect(create(:stage).teams).to match_array([])
+      end
+    end
+  end
+
+  describe '#over?' do
+    context 'group stage' do
+      context 'with unfinished matches' do
+        it 'returns false' do
+          expect(create(:group_stage).over?).to eq(false)
+        end
+      end
+
+      context 'with all matches finished' do
+        let(:finished_group_stage) do
+          group_stage = create(:group_stage)
+          group_stage.groups.map(&:matches).flatten.each do |m|
+            m.state = :finished
+            m.save!
+          end
+          group_stage
+        end
+
+        it 'returns true' do
+          expect(finished_group_stage.over?).to eq(true)
+        end
+      end
+    end
+
+    context 'playoff stage' do
+      context 'with unfinished matches' do
+        it 'returns false' do
+          expect(create(:playoff_stage).over?).to eq(false)
+        end
+      end
+
+      context 'with all matches finished' do
+        let(:finished_playoff_stage) do
+          playoff_stage = create(:playoff_stage)
+          playoff_stage.matches.each do |m|
+            m.state = :finished
+            m.save!
+          end
+          playoff_stage
+        end
+
+        it 'returns true' do
+          expect(finished_playoff_stage.over?).to eq(true)
+        end
+      end
+    end
+
+    context 'empty stage' do
+      it 'returns false' do
+        expect(create(:stage).over?).to eq(false)
+      end
+    end
   end
 end
diff --git a/spec/services/group_stage_service_spec.rb b/spec/services/group_stage_service_spec.rb
index b5ebf06..8073ef2 100644
--- a/spec/services/group_stage_service_spec.rb
+++ b/spec/services/group_stage_service_spec.rb
@@ -156,4 +156,38 @@ RSpec.describe GroupStageService do
       it_should_behave_like 'only_return_group_scores'
     end
   end
+
+  describe '#teams_sorted_by_group_scores' do
+    before do
+      @group_to_sort = create(:group, match_count: 10, match_factory: :filled_group_match)
+    end
+
+    let(:sorted_teams) do
+      GroupStageService.teams_sorted_by_group_scores(@group_to_sort)
+    end
+
+    let(:sorted_teams_grouped_by_group_points) do
+      sorted_teams.group_by { |t| @group_to_sort.group_scores.find_by(team: t).group_points }.values
+    end
+
+    it 'sorts the teams after group_scores first' do
+      i = 0
+      while i < (sorted_teams.size - 1)
+        expect(@group_to_sort.group_scores.find_by(team: sorted_teams[i]).group_points)
+          .to be >= @group_to_sort.group_scores.find_by(team: sorted_teams[i + 1]).group_points
+        i += 1
+      end
+    end
+
+    it 'sorts the teams after difference_in_points second' do
+      sorted_teams_grouped_by_group_points.each do |teams|
+        i = 0
+        while i < (teams.size - 1)
+          expect(@group_to_sort.group_scores.find_by(team: teams[i]).difference_in_points)
+            .to be >= @group_to_sort.group_scores.find_by(team: teams[i + 1]).difference_in_points
+          i += 1
+        end
+      end
+    end
+  end
 end
diff --git a/spec/services/playoff_stage_service_spec.rb b/spec/services/playoff_stage_service_spec.rb
index 97fdfb7..ee12487 100644
--- a/spec/services/playoff_stage_service_spec.rb
+++ b/spec/services/playoff_stage_service_spec.rb
@@ -70,7 +70,7 @@ RSpec.describe PlayoffStageService do
         amount_of_teams = parameters[:team_size]
         expected_amount_of_playoff_stages = parameters[:expected_amount_of_playoff_stages]
         teams = build_list(:team, amount_of_teams)
-        stages = PlayoffStageService.generate_playoff_stages(teams)
+        stages = PlayoffStageService.generate_playoff_stages(teams, false)
         expect(stages.size).to eq(expected_amount_of_playoff_stages)
         stages.each_index do |i|
           stage = stages[i]
@@ -82,7 +82,7 @@ RSpec.describe PlayoffStageService do
 
     describe 'number of teams isn\'t a power of two' do
       let(:generated_stages) do
-        PlayoffStageService.generate_playoff_stages(create_list(:team, 12))
+        PlayoffStageService.generate_playoff_stages(create_list(:team, 12), false)
       end
 
       let(:intermediate_stage) do
@@ -102,7 +102,7 @@ RSpec.describe PlayoffStageService do
 
     describe 'number of teams is a power of two' do
       let(:generated_stages) do
-        PlayoffStageService.generate_playoff_stages(create_list(:team, 16))
+        PlayoffStageService.generate_playoff_stages(create_list(:team, 16), false)
       end
 
       it 'generates only normal playoff_stage state stages' do