A review is presented of bioaccumulation of organic substances in organisms, especially fish, including the incentives for developing a tiered predictive approach for addressing the large number of chemicals of commerce. From a review of the existing estimation methods it is suggested that the simplest Tier 1 approach is an empirical correlation for bioconcentration factor as a function of the octanol-water partition coefficient. For more detailed Tier 2 evaluation, the bioaccumulation factor is best predicted using a mechanistic mass balance model applied to the organism at steady state in which relevant uptake and loss processes are quantified. The equivalence of rate constant and fugacity models is demonstrated and methods of obtaining parameter values are discussed. Such a model reveals the relative significance of gill ventilation, food uptake, egestion, and metabolism. The most detailed Tier 3 evaluation should involve prediction of the potential for biomagnification in a food chain involving both fish and air-breathing animals. Research needs are discussed with a view to understanding the mechanisms more fully and developing more accurate quantitative descriptions or models of bioaccumulation phenomena.