Because ecotoxicology deals with chemicals in the environment and how they affect organisms, a certain level of understanding of chemistry is needed to fully appreciate the science. However, many of the readers of this book may not have had a lot of (or any) formal training in chemistry, and others who have had some classes may benefit from a review of this field. In this chapter, I hope to provide enough chemistry so that the rest of this book makes sense, and I hope I don’t bore those who have had chemistry courses. We will very briefly cover the elements and the Periodic Chart. Then we will discuss how elements combine to form molecules and enter a cursory examination of the two major classes of molecules in ecotoxicology—metals and organics. Finally, we will review some concepts dealing with the fate and transport of chemical contaminants in the environment. If you have had training in chemistry and feel comfortable with the topic, you might want to skip over the parts you know well and focus on those sections that you may benefit from some refreshing. Students who desire greater information on basic chemistry may wish to consult one of the many texts on chemistry (e.g., Moore, 2011; Silberberg and Amateis, 2014).