Exosomes are small membrane vesicles of 30-100 nm in diameter and contain biological macromolecules such as proteins and nucleic acids. Exosomes are secreted from various types of cells and their characteristics depend on cell types. Exosomes are present in various body fluids including blood and urine. Recent research revealed that exosomes play important roles in intercellular communication by transporting their contents to exosome-recipient cells, so that the roles of exosomes in various biological events such as tumor metastasis and immune response have recently been attracted huge attention. As exosomes share the characteristics with their producing cells, exosomes are expected to be used as a biomarker for diagnosis. Moreover, several attempts have been made to use exosomes as drug delivery system (DDS). In this review, we first describe the characteristics and secretory pathway of exosomes. Then, the involvement of exosomes in biological events and application of exosomes to diagnosis and DDS is summarized. Besides, we introduce our recent results about the development of exosome-based DDS.