Abstract: Exosomes are a kind of endosomal vesicles that are secreted by most if not all living cells. Due to their capability of delivering a variety of cargos, such as tissue- or cell-specific proteins, lipids, and genetic materials, and their broad biological activities, exosomes have gained substantial attention as emerging therapeutics. Exosomes derived from mesenchymal stem cells (MSCs) and dendritic cells (DCs) are two types of exosomes that are widely studied. Many preclinical and clinical studies have shown that they have a satisfactory treatment effect in lung diseases, liver diseases, nervous system diseases, tumors, and other diseases. In addition, exosomes from macrophages, tumor cells, plant cells, and many other cells are getting more attention due to their therapeutic potential. Besides natural exosomes, research on engineered exosomes has also made plenty of progress. There have been several engineering methods of exosomes, such as targeting modification and loading of active ingredients. In this review, we summarize the research progress of therapeutic exosomes from different sources, and further discusses the application prospects of exosomes and possible challenges in the future.