When first developed, chlorinated pesticides such as DDT, dieldrin, and mirex were received with open arms, quickly becoming popular as effective, economic agents against pests. But evidence began to mount that residues of these chemicals remained in the environment, not breaking down, often appearing in plants and animals. By the late seventies many pesticides had achieved a terrible notoriety and were subsequently banned in a number of countries. Of tremendous concern, then, is the persistence of pesticides in the environment. The major thrust of research and development in the area of pesticides has properly been the creation of substances that are both effective and degradable. Yet in order to successfully promote the use of biodegradable pesticides, one must fully understand the mechanism of degradation, and it is to this vital subject that we address ourselves in the present volume. According to the Biodegradation Task Force, Safety of Chemicals Com­ mittee, Brussels (1978), biodegradation may be defined as the molecular degradation of an organic substance resulting from the complex action of living organisms. A substance is said to be biodegraded to an environmentally acceptable extent when environmentally undesirable properties are lost. Loss of some characteristic function or property of substance by biodegradation may be referred to as biological transformation.