Thousands of chemicals are in daily use; many of these reach the aquatic environment, which serves as a medium of transport and deposition into sediment and organisms. It is often unknown which effects these contaminants, alone or in combination with other stressors, may have on organisms, populations and communities in the aquatic environment. Ecotoxicology strives to identify and mechanistically understand contaminant–biota interactions with the ultimate goal to predict potential adverse effects. There are two general routes to addressing this goal. The first is to understand how organisms and populations respond according to anticipated exposure routes, organisms' genome and physiology, and structure- physico-chemical properties of chemicals. This knowledge should lead to improved criteria for chemical design and approval. The second general route is to identify the cause–effect relationships of effects detected in the environment without prior knowledge on the chemical's identity. This approach serves to pinpoint critical chemicals in complex environmental samples to enable decisions or technologies for their reduction or removal. An example will be presented in which biological effect assessment contributed to a better understanding of groundwater contamination.