Ecotoxicolgy and risk evaluation is a relatively young science becoming more and more important during the last decade. Ecotoxicology aims at characterising, understanding and predicting deleterious effects of toxic chemicals on biological systems. In this course a general theoretical introduction on ecotoxicology is given. Besides a historical summary, currently used notions are defined and relevant subjects are discussed briefly. To this end, it is also concerned with the fate of chemical substances in the environment, with factors that affect the chemical speciation and bioavailable portion of compounds and their uptake pathways into organisms, with metabolic activation and deactivation processes. Effects or interaction mechanisms on the suborganismic level, and with propagation mechanisms of biological effects on higher levels of organisation (organism, population, community, ecosystem) are discussed, along with their possible assessment methods. Besides a theoretical part 15 hours practical course is given, in which it is illustrated how combination toxicity of pollutants on aquatic organisms can be assessed.