Exosomes are membrane bound extracellular vesicles (EVs) that are produced in the endosomal compartment of most eukaryotic cells. The multivesicular body (MVB) is an endosome defined by intraluminal vesicles (ILVs) that bud inward into the endosomal lumen. If the MVB fuses with the cell surface (the plasma membrane), these ILVs are released as exosomes. In multicellular organisms, exosomes and other EVs are present in tissues and can also be found in biological fluids including blood, urine, and cerebrospinal fluid.