A brief history of metals, their early investigation in fish by physiologists and toxicologists, and current terminology are presented. The conceptual basis for the topics explored in each of the metal-specific chapters of these two volumes is then described. These include sources of metals, their economic importance, environmental situations of concern, essentiality or non-essentiality, bioconcentration or lack thereof, and the overarching importance of chemical speciation in understanding their effects on fish. The techniques used to derive ambient water quality criteria for metals are explained. Key mechanisms of acute and chronic toxicity are reviewed, as well as recent findings on the mechanisms and sites of uptake, internal handling, biotransformation, subcellular partitioning, detoxification, storage, and excretion. Important new research fronts focus on behavioral effects, molecular and omic analyses of cellular responses, and the effects of interacting metals in fish. Similarities and differences among the metals dealt with in these volumes are highlighted.