In the decade since the first edition of this book was published advances have been made in our knowledge of the fresh waters of the world, espe­ cially in understanding many of the processes involved in their functioning as systems and in countering the problems created by human activities. New problems too, many of an international nature, have loomed during this period-of which global warming and the acidification of fresh waters in many parts of the world are notable examples. In addition, much has now been published concerning the aquatic flora, fauna and ecology of previously poorly known geographic areas, notably Australasia. The second edition of this book is a revision which updates the text in the light of recent advances in our knowledge of freshwater biology. Inevitably, in an elementary volume such as this, the treatment of many of the basic principles and processes remains the same. However, several new sections are included covering a range of topics such as acid deposition and the acidification process, bacterial decomposition and aquaculture. The book includes many new references and suggestions for up-to-date reading in particular topics. The objective of the second edition remains the same as that of the first. It is intended as a basic introduction to the major aspects of freshwater biology at a level suitable for undergraduates. It should also prove useful, as apparently did the first edition, to professional workers in related fields, e.g. water engineers and chemists, aquaculturists and planners.