From a logical point of view, cell division is regulated by the environment and by the ability of the cell to respond to the environmental signals. The terminology of the cell cycle, the elaborate mathematical models, and the kinetic analyses are all convenient notations and descriptions of the behavior of populations of cells. However, they tell us very little about the fundamental molecular mechanisms that control cell proliferation. Stated in other terms, what controls cell reproduction are growth factors in the environment and genes and gene products inside the cell or at its surface. This book examines the aforementioned growth factors, the study of which has made very rapid progress in the past few years. The selection of topics has been influenced by logistic considerations, but the book, as a whole, gives a broad survey of the state of the art of this exciting field. For this, thanks are due to the contributors, who have given much time to the preparation of the manuscripts and have met the deadline with a punctuality that is uncommon among biomedical scientists. I would also like to thank Ms. NORA PERRETT and the staff of Springer-Verlag for their help in editing the manuscripts and in preparing the production of the book.