Toxicology is the study of toxic compounds and how these compounds can affect living organisms. In environmental toxicology we are especially concerned with how compounds are being distributed in the environment and taken up in the food chain, thereby affecting important components of the ecosystem. At the same time knowledge about how these compounds exert their mechanisms of action in a few species provides a basis for understanding effects in related groups. In this way fish, e.g. zebrafish, can be useful model organisms for understanding mechanisms that cannot be studied in humans or polar bears.