Bioaccumulation of pollutants can occur from sea water, from suspended particles, from sediments and through food chains. The rate at which accumulation occurs in an organism depends not only on the availability of the pollutant but also on a whole range of biological, chemical and environmental factors. The ultimate level which is reached is governed by the ability of the organism to excrete the pollutant or, alternatively, store it. This latter course often leads to the attainment of very high concentrations and sometimes no equilibrium level is ever reached. Two particular topics which are considered are the biological amplification of pollutants along food chains and the development of tolerance which sometimes occurs.