This chapter presents a reading of the National Liberation Army (ELN) and its evolution in Bolivia. Instead of extracting the essential facts from an standard corpus of publications and interviews that may fossilise the history of the movement, we will be tackling some aspects of what Hobsbawn may refer to as the formal social past, that is, the ELN’s dimensions that enable us to understand the present. To achieve this, we will outline the history of the guerrilla from its founding in 1967 to the 1990s, when, following a number of internal conflicts and reorganisations, it came together as a mass movement that would play a crucial role in the social movements that gave rise to the Movement for Socialism-Instrument for the Sovereignty of Peoples (MAS-IPSP), established in 1997 and that started to rule Bolivia in 2005.