Six basic mechanisms for heat exchanger fouling have been identified. In the accumulation of deposits in a particular heat exchanger, two or more of these mechanisms may be involved. It is a complex process. Fouling of heat exchangers leads to inefficient heat transfer with increased operating costs, and consequences for energy conservation and the environment. Mitigation of fouling begins with the appropriate design, with a recognition of the influence of design variables, velocity and temperature. Careful operation of the exchanger in line with the design criteria can assist fouling control, but the use of chemical additives or physical techniques will probably also be required for control. Off line cleaning will usually be needed.