Exosomes are small membrane vesicles, which eukaryotic cells secrete into their extracellular environment. They are formed as intraluminal vesicles by inward budding of the limiting membrane into the lumen of late endosomes. Upon fusion of thus arising multivesicular bodies with the plasma membrane, these vesicles are released as exosomes and enter body fluids such as blood plasma, urine and saliva. Containing certain combinations of lipids, adhesion and intercellular signaling molecules as well as RNAs, exosomes participate in intercellular communication processes. Depending on their origin, exosomes can modulate immune-regulatory processes, set up tumor escape mechanisms and mediate regenerative or degenerative processes, amongst others. In summary, exosomes are molecular complex intercellular signaling organelles with multiple functions, which appear as promising new tools for the clinical diagnostics and potentially for novel therapeutic strategies.