Exosomes have specialized functions and play a key role in different physiological processes and pathological conditions. Consequently, exosomes have attracted increasing attention in their clinical applications for prognosis, diagnosis, drug delivery, and vaccine development. Here, we give an introduction to the different exosome applications. Diagnostic Applications Biomarker: Exosomes released by cells into bodily fluids display different protein, lipid, and RNA contents in healthy individuals and patients with different diseases, indicating that exosomes can be utilized as potential diagnostic and prognostic markers. Moreover, the collection of exosomes from different samples such as blood, urine, and saliva represents minimally or non-invasive diagnostic approaches. Thus, exosomes serve as alternatives to traditional needle or excision biopsies due to the reduced patient pain and inconvenience, greater speed, and lower cost of analysis. It has been demonstrated that they could be used to determine the onset or progress of multiple diseases, including cancer, neurodegenerative diseases, cardiovascular diseases, pregnancy, infectious diseases, etc. Therapeutic Applications Drug delivery — As natural carriers of signaling molecules, exosomes are exploited for their potential in therapeutic delivery. After they are loaded with exogenous cargo molecules, exosomes can attach to target cells by a range of surface adhesion proteins and vector ligands, and deliver their payload to target cells. Exosome-based drug formulations may be applied to a wide variety of disorders such as cancer, various infectious, cardiovascular, and neurodegenerative disorders. Exosomes offer appealing features for drug delivery by providing the advantages of both synthetic nanocarriers and cell-mediated drug delivery: Biocompatibility Stability in the circulation Biological barrier permeability Low immunogenicity and toxicity