Building on recent studies of civilian militias outside the context of civil war and state guidance, this thesis examines the formation of Mexico’s autodefensas, self-defense militias which have risen in the wake of drug violence. Classifying militias by traits including use of violence and quasi-governmental activities, this analysis seeks to identify the primary factors that prompt their formation. My method considers over 30 states, collecting information from articles, reports, and national surveys to determine factors such as government attitudes towards militias, civilian attitudes regarding the formation of autodefensas, and territorial control. I assert that several factors provide conditions for autodefensa emergence, hypothesizing that autodefensas are more likely to form in the absence of a clear authoritative actor. Additionally, militias are more likely to arise when supported by civilians; finally, more autodefensas will form if territorial contestation exists, and the groups enjoy a positive relationship with the government. I find that government support is not influential in the formation of autodefensas. Contrarily, civilian support and territorial contestation show promise in explaining the formation of autodefensas. This paper provides guidance for research regarding civilian use of extralegal security measures, with implications for nations whose governments cannot provide security during violent times.