Aquatic animals are naturally exposed to a variety of metals whose chemical forms and concentrations are governed by natural geochemical processes and anthropogenic activities. These metals include both essential elements required to support biological processes and nonessential metals with no known biological function. Cellular functions are critical to processes involved in metal uptake, regulation, utilization, and release. Toxicity can be attributed to their dysfunction and the resultant interaction of metals with inappropriate cellular structures. Investigations at the cellular level will advance our understanding of the mechanisms by which aquatic animals respond to metal exposure.