The translation of a contaminant effect on the growth, behavior, or survival probability of individuals into a change in the size or productivity of a population involves the interspecies and intraspecies interactions that regulate population size. The result of these interactions may be to reduce or exaggerate the ultimate effect of the contaminant-induced stress and to cause the importance of the contaminant to vary with annual variations in the physical and biological environment. The interactions cause the density-dependent changes in growth, survival, and reproduction that are often associated with perturbations. These density-dependent responses of population characteristics confound the measurement of the population-level effects of contaminant stress in the field and their prediction on the basis of laboratory observations. For many, if not most, field situations the evaluation of population response necessarily involves the study of other biological and physical factors that affect population size.