Many hormones, growth factors, and other large molecules bind to speci- Jic receptors on the surface of eukaryotic cells and are rapidly taken into these cells. Current techniques of protein purification have made available sufficient amounts of these molecules so that detailed studies of their interaction with cells could be carried out. These studies have been performed on just a few types of cells, but it is clear that all types of cells carry out a_ similar internalization process. The realization that cells rapidly internalize hormones, growth factors, transport proteins, toxins, and viruses has led many investigators to address a similar series of questions: (1) What is the pathway by which macromolecules enter cells? (2) Do all macromolecules enter by the same pathway? (3) What is the function of internalization of large molecules? (4) What is the biochemical mechan- ism of internalization? In this volume we have tried to provide answers to these and related questions. To do this we have asked scientists currently active in the field to contribute chapters in their special areas of interest. The selection of the material covered reflects in large part areas of active research. Because of space limitations some important areas have not been covered as fully as we would have liked in this volume, but will be covered in a future volume. Our aim has been to present a consistent view and, when disagreements exist, to point out the basis of such disagreements.