The physiological characteristics of the environmental interface of organisms determine the metal forms of highest bioavailability. Studies of metal uptake from solution by aquatic organisms verify the high availability of free metal ions. Metals also are accumulated from food by many aquatic organisms, as indicated by both laboratory and field studies. The quantitative importance of the food vector depends upon biological availability, which differs with the specific type of food being ingested. Uptake from both food and solute vectors may be influenced by interactions among cations, pH, redox, temperature and physiological variables. Separating their relative importance through a basic understanding of these processes will be a necessary prerequisite to understanding metal impacts in natural systems.