Cells of multicellular organisms exchange informative signals by diverse mechanisms. Recent findings uncovered the special role of extracellular vesicles, especially exosomes, in intercellular communication. Exosomes, present in all tested human bodily fluids, carry various functional compounds including proteins, lipids, and diverse RNA molecules. The composition of exosome cargo in vivo is likely formed by a regulated selection of specific components and can express the current status of the exosome-secreting cell. Therefore, particular emphasis is now placed on the extremely high potential of exosomes as essentially noninvasive prognostic and diagnostic biomarkers, but also as therapeutic nanocarriers, especially after the discovery that their cargo as well as cell-targeting specificity could be shaped in vitro. In addition, targeting the exosomes mediating pathological intercellular communication may also express high therapeutic potential. Hence, numerous studies are conducted to explore the profile and function of exosomes and their cargo in health and disease and to shape their properties to facilitate their clinical application. The present review summarizes the current knowledge on the role of exosomes in different physiological and pathological mechanisms of intercellular communication with a particular focus on the use of exosomes in the diagnosis and treatment of various inflammatory, cardiovascular, metabolic, and neurodegenerative disorders as well as malignant neoplasms.