Unlike laboratory animals, people are rarely exposed to a single hazardous chemical. However, most of the information documenting adverse human health effects from environmental and occupational contaminants has come from studies focused on exposure to single chemicals, and there is little information available on how two or more contaminants affect humans. Most information on the effects of mixtures comes from animal systems and limited investigations of isolated human cells in culture, even though the study of mixtures in such systems has also been neglected. Two or more compounds may show additive, antagonistic, or synergistic interactions or may act on totally different systems and thus not interact. Furthermore, even a single chemical may have multiple effects and affect more than one organ system. Effects may vary with age, and metabolites may have totally different actions from the parent compound. This paper will review the variety of health effects in humans that may result from environmental contaminants and discuss how such contaminants may interact with each other. We will also present examples on how different contaminants interact from toxicologic studies of polychlorinated biphenyls performed as part of our Albany, New York, Superfund Basic Research Program project.