In the early days of ecotoxicology, researchers tested the effects of nominal concentrations in static systems. Basically, a pollutant was added to a test system, such as an aquarium with fish, until a clearly distinguishable effect was observed, such as death or immobilization. As the discipline has developed, the scope of projects has become more sophisticated. Now, researchers have moved from establishing baseline data sets to distinguishing between nominal, free, internal, and target concentrations. In the following discussion, we will review these developments and show how internal exposure data could improve risk assessment and bridge the gap between human health and environmental risk assessment.