The San Pedro River is the fluvial system being drained by the chain of eight Andean lakes in the Valdivia River Basin (de los Rios Region). This system covers 40 km from the outlet of Rinihue Lake to its confluence with the Collileufu River, where it is renamed as Calle Calle River. In the San Pedro River, the "Central Hidroelectrica San Pedro" (San Pedro Hydroelectric Plant) is planned, whose dam is located 14 km downstream from the outlet of the Rinihue Lake, which will transform 12.5 km of river in a reservoir. This generated the need for an Environmental Impact Assessment, which was developed by the Center for Environmental Sciences EULA-Chile (Universidad de Concepcion). In relation to the river ecology, the study placed special emphasis on generating detailed knowledge about their ichthyofauna, in order to make a species-specific impact assessment. In this issue, we present the foundations and methodological approaches to undertake the intensive study, whose results give rise to this monograph and to the description of the distribution, abundance, habitat use, movement patterns, length-weight relationships, early life stages and genetic structure of the fish fauna of San Pedro River.