Most of my career as a social psychologist has been devoted to the study of adult social behavior. The shift to children's friendships was a new enterprise, one that I could not have carried out successfully without a great deal of help from others. The Foundation for Child Development, the National Institute of Mental Health, and the Social Science Research Council provided funds that enabled me to spend the year 1977-78 at the Institute of Human Development at the University of California, Berkeley, where I immersed myself in the social world of young children and began to write this book. At Berkeley I received invaluable assistance from many researchers, staff members, and nursery school teachers, including especially Paul M�ssen, Jane Hunt, Hannah Sanders, Barbara Scales, and Louise Singleton. In the preparation of this book I received continued financial assistance from the Foundation for Child Development, accompanied by the moral support of Orville G. Brim, Jr., and Heidi Sigal at the Foundation. I owe a special debt to Peggy Stubbs, who graciously made available to me the extensive logs about friendships that older children had written for her when she was their teacher. Among the many colleagues who have provided unpublished materials and have commented on portions of the manuscript, I am especially indebted to Gary Fine, Joseph Jacobson, Mary Cover Jones, Michael Lougee, and Elliott Medrich. Once again, I am grateful to Carol Rubin, for her valuable comments on the manuscript, her encouragement, and her love. Finally, I would like to express my thanks to the nursery school children at the Harold E. Jones Child Study Center in Berkeley and to Elihu Rubin and his friends in Berkeley and Boston, for letting me observe the development of their friendships.