Extracellular vesicles (EVs) are increasingly recognized as active mediators of intercellular communication rather than inert cellular byproducts. This chapter presents a comprehensive overview of EVs, encompassing their biogenesis, classification, molecular composition, and functional roles across physiological and pathological states. By examining the distinct origins, formation pathways, and cargos of exosomes, microvesicles, and apoptotic bodies, it highlights how EVs constitute a complex and regulated system of intercellular signaling. EVs transport diverse molecular payloads, proteins, lipids, mRNAs, miRNAs, and other noncoding RNAs, facilitating functional modulation of recipient cells. These vesicles play crucial roles in immune regulation, tissue homeostasis, neurogenesis, and tumor progression, and serve as key effectors in processes such as inflammation, angiogenesis, and metabolic adaptation. Acting both locally and systemically, EVs enable context-dependent modulation of cell behavior, establishing them as essential components of multicellular communication networks. The discussion extends beyond cellular biology to considering the translational promise of EVs. Increasingly, EVs are being investigated as minimally invasive biomarkers due to their stability in biofluids and ability to reflect the molecular state of their cells of origin. In clinical fields such as oncology, neurology, and autoimmune disease, EVs offer molecular snapshots of disease progression, enabling early detection, real-time monitoring, and potential prognostic evaluation. Concurrently, the intrinsic stability, low immunogenicity, and targeting capabilities of EVs position them as compelling candidates for therapeutic delivery systems. Strategies are being developed to engineer EVs for the transport of small RNAs, CRISPR-Cas components, and immunoregulatory molecules, exploiting their native tropism and biocompatibility. However, the field faces unresolved challenges, including heterogeneity in EV subtypes, lack of standardized isolation methods, and limitations in delivery efficiency and scalability. Technological innovations in microfluidics, high-resolution profiling (including proteomics and transcriptomics), and synthetic biology are rapidly expanding the toolkit available for EV characterization and manipulation. These advances are poised to accelerate the clinical translation of EV-based diagnostics and therapeutics. By positioning EVs as both messengers and modulators, the chapter situates them at the intersection of basic science and applied medicine. As the mechanistic understanding of EV biology deepens, these vesicles are poised to shift from observational biomarkers to active agents of precision therapy, underscoring their potential to reshape the landscape of personalized medicine.