Rapid progress in the field of sialic acids has made it desirable to collect the new data about these unique sugars and to continue the series of books on this topic. In 1960, A. GOTTSCHALK wrote 'The Chemistry and Biology of Sialic Acids and Related Substances' (Cambridge University Press) and in 1976, A. ROSENBERG and C. -L. SCHENGRUND published 'Biological Roles of Sialic Acids' (Plenum Press). In this book emphasis is given to various modern methods used in the isolation and analysis of sialic acids. New approaches to the synthesis of free and bound sialic acids are described and the vast field of occurrence and metabolism of these substances is reviewed. Sialidoses are dealt with in one of the chapters, because sialidases have been recognized as factors of pathophysiological im portance. As knowledge is increasing about the involvement of sialic acids in many aspects of cell biology, another chapter is devoted to these phenomena. With this book I intend to demonstrate modern trends in sialic acid chemistry and biochemistry, and I hope that it will be of practical use and find its place in laboratories rather than in libraries. This publication offers an opportunity to thank all colleagues in many countries, including my coworkers at the universities of Bochum and Kiel, for their cooperation, stimulating discussions and, very important, useful criticism. The continuous cooperation with J. F. G. VLIEGENTHART and his coworkers, Utrecht, has been rewarding in many respects.