Grass is a very important world crop. In some countries, for example the UK, Australia and New Zealand, animal products from grassland make a greater contribution to the value of agricultural production than does any other crop. Yet research being undertaken to further· our understanding of the factors affecting the growth and productivity of grasslands has trailed in the shadow of the determined efforts made to improve our knowledge of cereals and, to a somewhat lesser extent, legumes. However, in spite of its low profile, grassland research has resulted in considerable advances in our knowledge in the last 20 years, and we feel that this book provides a timely opportunity to bring together some of this work in a review of what is primarily the ecophysiology of the temperate grass crop. Unlike other crops grown for their grain or vegatative parts, grass and grassland products are used almost entirely for the feeding of ruminant animals; the interaction of the sward and the animal thus adds an extra dimension to investigations of the productivity of grassland. No one author could adequately encompass the breadth of work covered in the book. Acknowledged experts have therefore been selected as contributors to provide an up-to-date review of their own specialized areas. Whilst multi author texts can cause problems of lack of uniformity of approach, each contributor has been made aware of the contents of the other chapters in an attempt both to provide continuity and to prevent glaring overlaps.