Environmental monitoring activities in Chile are relatively new and have traditionally relied on physicochemical measurements. The pulp mill industry in Chile is highly competitive in the global market and several new large mills have recently opened. Early studies on fish in the receiving environments revealed lower species richness and an increase in the abundance of introduced species relative to native ones near pulp mill discharges. Even though changes were observed, their relationship with the discharges was unclear. Several difficulties related to small body sizes and the unavailability of basic biological data for native Chilean fish species were found during initial field studies. One of the main challenges is the standardization of monitoring methods (including fish species selection, sampling sizes, indicators, reference sites, etc.) and consensus about the responses that should be considered in a river monitoring program in the Chilean context. This paper summarizes major findings from a series of studies looking at impacts on fish at different levels of biological organization and the current approach used in Chile for monitoring impacts of pulp mill effluents on wild fish populations.