This article discusses the use of extracellular vesicles (EVs) for drug delivery and theranostics in vivo. EVs are lipid bilayer-enclosed nanopouches that are abundant in body fluids and can exchange various constituents between cells. They have low zeta potentials and short circulation times but offer low immunogenicity and inherent stability. The article highlights the challenges in EV clinical application, such as controlling immune system clearance, large-scale production, and understanding in vivo mechanisms. It also explores strategies to overcome these limitations. The article describes the classification of EVs into microvesicles and exosomes, their biogenesis, and the diversity of their contents. It discusses methods for isolating EVs, including ultracentrifugation, density gradient separation, and chromatography, and the need for combining multiple methods for effective isolation. The article also covers the contents of EVs, such as proteins, lipids, and nucleic acids, and their potential for drug delivery. It compares EVs with liposomes, emphasizing the advantages of EVs, such as nonimmunogenicity and better drug delivery efficacy. The article outlines engineering approaches for EV-based drug delivery, including genetic, biochemical, and nanomaterial-based methods. It also discusses challenges in drug loading and the potential of autologous EV administration in personalized medicine. The article highlights the use of EVs in theranostic applications, combining drug delivery with imaging modalities, and provides examples of EV-based therapies for various diseases. It concludes with the future prospects and challenges in EV research, emphasizing the need for standardized protocols, safety assessments, and further clinical trials.This article discusses the use of extracellular vesicles (EVs) for drug delivery and theranostics in vivo. EVs are lipid bilayer-enclosed nanopouches that are abundant in body fluids and can exchange various constituents between cells. They have low zeta potentials and short circulation times but offer low immunogenicity and inherent stability. The article highlights the challenges in EV clinical application, such as controlling immune system clearance, large-scale production, and understanding in vivo mechanisms. It also explores strategies to overcome these limitations. The article describes the classification of EVs into microvesicles and exosomes, their biogenesis, and the diversity of their contents. It discusses methods for isolating EVs, including ultracentrifugation, density gradient separation, and chromatography, and the need for combining multiple methods for effective isolation. The article also covers the contents of EVs, such as proteins, lipids, and nucleic acids, and their potential for drug delivery. It compares EVs with liposomes, emphasizing the advantages of EVs, such as nonimmunogenicity and better drug delivery efficacy. The article outlines engineering approaches for EV-based drug delivery, including genetic, biochemical, and nanomaterial-based methods. It also discusses challenges in drug loading and the potential of autologous EV administration in personalized medicine. The article highlights the use of EVs in theranostic applications, combining drug delivery with imaging modalities, and provides examples of EV-based therapies for various diseases. It concludes with the future prospects and challenges in EV research, emphasizing the need for standardized protocols, safety assessments, and further clinical trials.