Although levels of violence in Mexico have generally decreased, since 2013 many self-defense groups have emerged across the country. Yet, little is known about the impact these militias have on levels of criminal violence. The purpose of this thesis is to contribute to the understanding of the impact militias have on violence outside civil wars. Through a case study comparing two sta tes in Mexico where selfdefense groups have gained importance, this thesis aims to uncover whether the development of selfdefense groups in the states of Michoacán and Guerrero has impacted the levels and types of violence during 2013-2014. Self-defense groups are found to indirectly impact violence in Mexico. Defensive violence by self-defense forces has partially interrupted the illicit activities of drug trafficking organizations in the areas under the control of self-defense groups. Self-defense groups played a crucial role in urging the government to take swift action; hence, their presence can cause more enforcement violence. Together enforcement and defensive violence can diminish the capabilities of drug trafficking organizations and thus lead to competitive violence between drug cartels. While the cooperation of government and militia can be helpful, a lasting peace can only be achieved if drug trafficking organizations stop competing for control over lucrative illicit businesses.