The problem of preventing mass atrocities (such as genocide, war crimes, crimes against humanity and ethnic cleansing) has beset the international community, especially since the end of the Second World War in 1945. The mass killing of the Jews during the Holocaust scarred humanity’s conscience and led to the promise that ‘never again’ will the international community, represented by the United Nations (UN), stand idly by while these crimes are happening. Nevertheless, the UN failed to fulfill this promise in Rwanda in 1994. Since then, regional organizations, particularly the African Union (AU), have become increasingly involved in mediation efforts to prevent these crimes, such as in Sudan, Kenya, Guinea, Cote d’Ivoire, Central African Republic, Burundi, and South Sudan – to mention a few. Despite the presumed effectiveness of regional organizations in resolving regional and local conflicts, these mediation efforts have resulted in different outcomes, where some succeeded and other failed, thus raising questions over the AU’s effectiveness.To examine and explain the success and failure of AU-led or assisted mediation in atrocity crime conflicts, I adopt a framework for analysis that focuses on the concepts of the responsibility to protect (R2P) and regionalism (the mediator). Also, the framework centres on mediation success variables of the mediator characteristics (legitimacy), behaviour (strategy) and the nature of parties (interests). Additionally, the framework involves the factors scholars associate with success of mediation for atrocity crimes prevention, such as single and early mediation, unity of international actors and coercion. I utilize qualitative research and structured, focused case study comparison method and evaluate the cases of South Sudan (2014-2015), Kenya (2008) and Guinea (2009-2010).I argue that there is a paradox in AU mediation for atrocity crime prevention: while the AU’s legitimacy ensured the consent of the parties and early mediation, its strategy was not coercive, and this led to ineffectiveness, lack of ripeness and failure. AU mediation succeeded when its nominated mediator or joint mediators utilized a coercive strategy, particularly the simultaneous use of sanctions and judicial prosecution. The absence or presence of a coercive strategy explained the failure and success of AU mediation. AU mediation followed established principles of mediation in armed conflicts, including consent, voluntariness and consensual agreement. The AU’s choice of strategy was the dominant factor for the success or failure of its mediation.Given the UN’s growing attention on the AU as the locus for future preventive actions and the use of peaceful mechanisms, as opposed to the use of force, this thesis contributes substantial knowledge for understanding and engaging the AU in mediation interventions in Africa.