Provided here is an updated and comprehensive view of basic and applied aspects of endocytosis. The first section covers new developments obtained by molecular tracking of defined probes in microcinematography and high-resolution electron microscopy, new procedures in cell fractionation, cell-free assays and genetic engineering of receptors transfected into eucaryotic cells. It also describes endocytosis in non-mammalian cells. The second section explores several applications to the understanding of health and disease, especially in the immune response and the relations between cells and intracellular pathogens, including in immunodeficient patients. It also discusses toxins and explores their potential for immuno-toxins, as well as the development of better designed drugs.