This book, about biological matters determining the quality and usefulness of Australian fresh waters, was written for several reasons. It was written because Australian fresh waters have many distinctive biological and other features, and therefore overseas work and management of water resources is often not relevant and must be used with caution in Australia. A main aim of the book, therefore, has been to aid Australian water management authorities by presenting useful ecological and other biological knowledge on Australian fresh waters. Another reason was to bring together in one volume such knowledge since hitherto this has been scattered, often somewhat inaccessible and sometimes out of date. Since freshwater resources are not abundant in Australia, it is important that they should be well managed. This book aims to help all those concerned with water management to achieve the best management possible. It will be of interest also to other authorities and all concerned with preserving Australia's water resources.