Extracellular vesicles (EVs) are the substances that are conveyed by most sorts of cell, which play a critical role in cell-to-cell communication. Exosomes are among the most commonly explored EVs. Exosomes contain lipids, RNAs and proteins. Many examinations have shown that exosomes derived from cells play different biological roles in normal as well as diseased condition. Recent investigations have explored exosomes as natural drug delivery vehicles, as they have ability to deliver different cargo to nearby and targeted cells with high specificity and efficiency. In the last decade, there has been increased interest in exosomes research due to their potential role in disease diagnosis as intercellular messengers. Exosomes have potential in targeted drug/gene delivery, due to their unique properties, such as low immunogenicity, excellent tissue/cell penetration capacity and innate stability. As a result, exosomes can be used for disease detection and for various treatments. In this review, we focus on the potential use of exosomes as valuable diagnostic and prognostic biomarkers for their cell-lineage and state-specific contents, and their possible use as therapeutic vehicle for drug and gene delivery. As a drug, exosomes have a variety of applications in various diseases.