Endocytosis is the process by which cells absorb molecules (such as proteins) from outside the cell by engulfing it with their cell membrane. It is used by all cells of the body because most substances important to them are large polar molecules that cannot pass through the hydrophobic plasma membrane or cell membrane. Endocytosis is required for a vast number of functions that are essential for the well being of a cell. It intimately regulates many processes, including nutrient uptake, cell adhesion and migration, receptor signalling pathogen entry, neurotransmission, receptor down-regulation, antigen presentation, cell polarity, mitosis, growth and differentiation, and drug delivery. This book examines the functions and benefits of endocytosis in regards to cells within the body.