Unlike the highly visible and obvious effects caused by oil, certain chemicals have unseen but long-term and far-reaching effects on the marine ecosystem. They often have long half-lives, are carried great distances and pass easily through the food chain from prey to predator. The behaviour and effects of these pollutants on each type of marine animal (invertebrates, vertebrates, fish, mammals and fish-eating birds) are described, and an overview provides an understanding of their behaviour in the marine ecosystem.