The preface of a book often provides a convenient place in which the author can tender his apologies for any inadequacies and affords him the facility to excuse himself by reminding the reader that his art is long but life, or at least the portion of it in which he has the opportunity for writing books, is short. I, too, am deeply conscious that I have undertaken a task which I could not hope to complete to my own satisfaction but I offer, in self­ defence, the observation that, inadequate though it is, there is no other book extant, so far as I am aware, which provides the information contained herein within the covers of a single volume. Often during the last decade, in discharging my responsibilities for the environmental aspects of the water authority's operations and works, I should have been deeply grateful to have had access to a compendium such as this. The lack of a convenient source of data made me aware of the need which I have attempted to fill and in doing so I have drawn on my experiences of the kinds of problem which are presented to biologists in the water industry. The maxim 'half a loaf is better than none' seems particularly apt in this context.