It was the author's privilege to hold the George Fisher Baker Nonresident Lectureship in Chemistry at Cornell University during the spring of 1948. This book had its inception at that time. The uncompleted manuscript was laid aside shortly thereafter, and the work was not resumed until 1951. During the intervening years much new material became available from the ever-increasing literature, and major revision of manuscript and outlines consequently was necessary. The momentum of extensive investigations on the kinetics of addition polymerization, undertaken in various laboratories at the close of the war, carried this phase of the subject to an advanced stage of development. The material on dilute polymer solutions, appearing in the later chapters, had to be completely recast and expanded in the face of recent theories and experiments. The book consequently goes considerably beyond the Baker lectures both in scope and in the inclusion of new developments.