Plant hormones play a crucial role in controlling the way in which plants grow and develop. While metabolism provides the power and building blocks for plant life, it is the hormones that regulate the speed of growth of the individual parts and integrate them to produce the form that we recognize as a plant. This book is a description of these natural chemicals: how they are synthesized and metabolized, how they act at both the organismal and molecular levels, how we measure them, a description of some of the roles they play in regulating plant growth and development, and the prospects for the genetic engineering of hormone levels or responses in crop plants. This is the third edition of the highly acclaimed text/monograph last published in 1995 under the title "Plant Hormones: Physiology, Biochemistry and Molecular Biology". Two-thirds of the thirty-one chapters, written by a group of fifty-two international experts, are totally new, and all other chapters have been completely rewritten to include the latest information on Plant Hormones, particularly with reference to such new topics as signal transduction, brassinosteroids, responses to disease, and expansins. The book is not a conference proceedings but a selected collection of newly written, carefully integrated and illustrated reviews describing our knowledge of plant hormones and the experimental work that is the foundation of this information.