One of the most demanding challenges in the transition from war to sustainable peace is the transformation of non-state armed groups into civilian, accountable entities. Drawing on detailed research of the Micro Politics of Armed Groups Research Group at Humboldt University, the authors analyse several factors that have a direct impact on successful transition, most notably organisational structure and history. From Guerrilla War to Party Politics explores two successful, yet very different examples of successful post-war transformation of non state armed groups into political parties, the FSLN in Nicaragua and the FMLN in El Salvador.