The intent in initiating this volume was to bring together a series of essays which would define our present understanding of the endosome and lysosome and their interrelationship. The editors deliberately encouraged the contributors to be speculative to strive to put order to the real world of incomplete and sometimes conflicting data. Seeing science from the laboratory bench can often be like viewing an impressionistic painting from up close a series of paint dabs with no apparent order. The contributors to this volume were asked to step back and leave the reader with a sense of the whole as well as the detail. To the extent that this has happened, the credit should go to the individual authors. brOur understanding of endosomes and lysosomes has undergone a molecular revolution over the last decade. Hence, we now know much about the molecular features required for internalization of an endocytic receptor, or the function of mannose 6-phosphate receptors in the transport of lysosomal