Pesticides are toxic substances that are deliberately released into our environment to kill organisms which, from the point of view of man, are undesirable. The perfect pesticide would eliminate the target organism and be otherwise harmless. Unfortunately, as Dr Wurster and others have shown, some pesticides become environmental contaminants and have harmful effects that are often distant in time and space from the place where they were applied. These are calculated risks which have to be taken when the pesticide is used; the user believes that the advantages of application outweigh the disadvantages when, or perhaps more particularly if, he himself will not suffer from the side-effects of his actions. My present purpose is to deal with some of the other chemicals which man may introduce into his environment—usually accidentally, but sometimes deliberately, as in the cases of wastes discharged into the air or the sea as a method of disposal— which can also be the cause of environmental degradation.