This book is in many ways a sequel to The Biochemistry ojGlycoproteins and Proteoglycans. The enormous recent progress in understanding the biological roles of glycoproteins has prompted the present volume. The reasons for studying glycoproteins have multiplied, and in the present volume the roles played by glycoproteins are explored in a variety of biological situations. The first two chapters describe molecules involved in cell-substratum and cell-cell interactions in a broad sense, and also focus on recent progress in identifying specific attachment molecules. Our understanding of how normal processes, such as cellular differentiation and tissue organization, are regulated is dependent on understanding how cells interact with the extracellular matrix. When these processes go awry the consequences can be tragic, for example, when manifest as birth de­ fects and cancer. Our ability to devise appropriate therapies is in many cases limited by our understanding of such cell-matrix interactions. The third chapter explores the roles by glycoproteins during early mammalian development. The carbohydrate portions c1early play very important roles in presenting information during early embryogenesis, and an unusual tumor stern cell, the embryonal carcinoma, looks very promising in pro­ viding an experimental system for understanding how the expression of these complex carbohydrate determinants is regulated. The next three chapters explore the biology of glycoproteins in distinct situations: in the immune system, in the nervous system, and during erythropoiesis.