Endocytosis {Endo (within) cytosis (cell) } is a process in which a substance gains entry into a cell without passing through the cell membrane. This process is subdivided into three different types:

In each case endocytosis results in the formation of an intracellular vesicle by virtue of the invagination of the plasma membrane and membrane fusion. The different catagories differ in the exact mechanisms by which this process occurs.