Exosomes are extracellular vesicles released from living cells in an energy-dependent process. Exosomes are specifically loaded with nucleic acids, lipids, and proteins from their parental cell. Therefore, the constitution of extracellular vesicles reflects the type and status of the originating cell. Exosomes are secreted by many cell types into diverse body fluids such as blood, semen, urine, saliva, breast milk, ascites fluid, and cerebrospinal fluid. The main difference to other extracellular vesicles such as apoptotic vesicles or membrane vesicles is the endocytic origin of exosomes. Exosomes are released from intact cells after inward budding of multivesicular bodies and fusion with the plasma membrane. They have the same membrane orientation as the originating cell, i.e., displaying extracellular domains on their surface. CD9, CD63, and CD81 are three of the most-studied members of the tetraspanin protein family and can be used to isolate exosomes.