The evolution of the criminal gangs and militia groups operating in Medellin, Colombia, today mirrors the evolution of tensions and disputes in the city over time. Research on their origins and dynamics reveals the borrowing of revolutionary ideology from the guerrillas and the emergence of groups that combine the features and behaviors of the two kinds of groups. Reporting on this research, this article briefly outlines the background to armed conflict in Colombia, presents an analysis of Medellin as an instance of widespread armed conflict, outlines the collective actors at the barrio level from an evolutionary perspective, and then examines the recent changes among these actors and discusses the future directions of armed conflict in Medellin.