In December 2005, the United Nations established its Peacebuilding Commission (PBC), whose purpose was to bring together all relevant peacebuilding actors (international and regional organisations, local authorities and civil society, and UN organs) in order to increase the coordination between them, to marshal resources, and to establish integrated strategies. The creation of the PBC was based on the UN recognition of its lack of capacity to help post-conflict states to build a sustainable peace. Since 2006, four cases have been included in the PBC''s agenda — Burundi, Sierra Leone, Guinea-Bissau, and the Central African Republic. Furthermore, the organ created a Working Groups On Lessons Learned to develop best practices on peacebuilding. Therefore, this MA dissertation submitted to the London Metropolitan University: (1) reviews the UN''s peacebuilding historical record; (2) analyses the PBC''s structure; (3) discusses the PBC''s engagements in the four cases; (4) explores the PBC''s lessons learned on peacebuilding; and finally (5) evaluates whether or not the PBC has improved UN peacebuilding capacity.