A dictionary of a living language does not seek to lay down arbitrary laws dictating the sense in which words and phrases are to be used. It is a record, no more, of the way particular expressions are being used at a particular time it is as alive, or as dead, as a language at records. In most scientific disciplines, developments are occurring so rapidly, new terms are being coined — some unnecessarily — at such a rate, that the collection and arrangement of them presents very real problems. Nowhere is this more true than in the environmental sciences. Still emerging, they do not even have a name of their own to describe them, for "ecology," the word most commonly used, has a special limited meaning of its own. The professional ecologist today must be part of botanist, zoologist, chemist, physicist, mathematician, geologist, meteorologist, statistician, geographer and, if he finds himself drawn into the wider environmental movement, economist and philosopher as well. He will find himself confronted with words drawn from all of these disciplines, and probably more. In this dictionary I have attempted to draw from as wide a range of subject areas as I can but, inevitably, the final decision, the allocation of priorities, is mine and it is arbitrary.