Since effects of contaminants are often a consequence of concentrations in target organs or tissues, it is important to understand and to be able to predict the accumulation of contaminants in biota. Also, because human exposure commonly occurs through the consumption of tainted food, the ability to predict the amount of contaminant accumulating in these potential vectors of exposure is crucial to avoiding human poisonings. Bioaccumulation is the net accumulation of a contaminant in, and in some special cases on, an organism from all sources including water, air, and solid phases in the environment. Often bioaccumulation models are fit to the curve of concentration in the organism versus time of exposure using nonlinear regression methods. Although rate constant-based models have the longest history in ecotoxicology, clearance volume-based models have much promise in allowing linkage to the pharmacokinetics literature and techniques.