This chapter explores the biological processes that will be discussed with respect to four major classes of marine contaminants: heavy metals, artificial radionuclides, chlorinated hydrocarbons, and petroleum hydrocarbons. Obviously, much more is known about biological transport for some types of contaminants than others, and this is reflected by their treatment. The absorption of pollutants from food takes place in the gut with transport to the various tissues via the circulatory system. Absorption and tissue distribution of ingested metals and radionuclides depend on the bioavailability of the element. Very few studies have specificially examined the relative importance of food and water in the accumulation of hydrocarbons by marine species. In most long-term elimination studies with fish and other organisms, loss rates for petroleum hydrocarbons diminish with time. The low degree of assimilation of ingested petroleum hydrocarbons in deposit-feeding worms indicates that fecal casts will be instrumenta in redistributing this group of compounds in the sediments.