Exosomes are extracellular vesicles with a diameter of 50-150 nm and are secreted by most cell types. Exosomes are formed within the late endosomes by invagination of the membrane, leading to the formation of multivesicular bodies. Upon fusion of the multivesicular bodies with the cell membrane, exosomes are released into the extracellular space. Thus, exosomes exist in body fluids, including blood, urine, and cerebrospinal fluid. Exosomes encapsulate various proteins and nucleic acids such as miRNA, mRNA, and DNA that are contained in the cells from which exosomes originate. Because the molecular signature of exosomes reflects that of the cells from which they originate, exosomes have been proposed to be a novel type of biomarker for various diseases, including cancer. Detection of cancer-specific exosomes in blood could be useful for diagnosis and prognosis of cancer. It has been recently demonstrated that CD147 embedded in cancer-linked exosomes circulating in the blood can be used for detection of colorectal cancer. Moreover, exosomes can mediate intracellular communication and are transported to the recipient cells where cargo such as miRNA and mRNA, encapsulated within the exosomes, exert physiological or pathological effects. Exosomes released from cancer cells influence several aspects of cancer progression such as microenvironment, angiogenesis, apoptosis, and immunosurveillance, thereby promoting their survival. Furthermore, exosomes can be used as tools for drug delivery. Some reports have shown that exosomes with artificially introduced nucleic acids or drugs are effective in cancer treatment. In summary, exosomes play multiple roles in physiological and pathological conditions and can be used as biomarkers and therapeutic targets.