Extracellular vesicles (EVs; includingclassical-exosomes, non-classical-exosomes, microvesicles, large oncosomes, apoptotic bodies, apoptotic vesicles, autophagic extracellular vesicles, amphisomes and ARRMs) are membrane vesicles of 40-1000 nm that are released into the extracellular milieu and body fluids from most cell types, including red blood cells, platelets, lymphocytes, dendritic cells, endothelial cells and tumor cells. These vesicles are classified into 2 types according to their secretory mechanism. Thus, classical-exosomes are formed in multivesicular endosomes, whereas microvesicles originate by direct budding from the plasma membrane. Although classical-exosome components vary by their originating cell type, a certain set of molecules appears likely to be shared, regardless of their origin. These molecules include the tetraspanin proteins CD9, CD63 and CD81; these proteins are thought to be essential components of the biogenesis mechanism of classical-exosomes. Accordingly, researchers have used CD9, CD63 and CD81 to isolate and characterize the purity of classical-exosome preparations.Recently, exosomes have been discoveredin the milk of avariety of mammals, including human. It was shown that several exosome markers, such as CD63 and CD81, were also expressed in human breast milk. It has been reported that human breast milk contains many microRNAs, especially those involvedin immunity. It was suggested that these exosomalmicroRNAs might be transferred from mother to infant through breast milk. It has been reported that immunocytes areregulated by exosomes present in human breast milk. It has also been reported that exosomesderived from pig breast milkexhibited a growth-stimulating effectdirectly upon infant intestinal epithelial cells. Because of thesestudies, the function and mechanisms of action of exosomes in breast milk have attractedconsiderable attention.