As illustrated in Fig. 1, there are three distinct modes of entry into the lysosomes. The main route is by endocytosis, which depends on enclosure within vacuoles derived from invaginations of the plasma membrane. In most cases, these vacuoles subsequently fuse with lysosomes, either primary or secondary, and thus become part of the lysosome system. The materials that can be taken up in this manner include a variety of small molecules, all major groups of macromolecules, insoluble particles, as well as such complex objects as viruses, bacteria, parts of other cells, and even whole cells. There is thus practically no limit to what can be introduced into lysosomes and this unique accessibility is one of the features that make these particles an ideal target for drugs. Another useful property ofendocytosis is its specificity. Different materials are taken up at different rates, and these rates vary considerably, both in absolute magnitude and relative to each other, from one cell type to another.