<p>Over the past few decades, extensive basic, translational and clinical research has been devoted to deciphering the physiological and pathogenic roles of extracellular vesicles (EVs) in the nervous system. The presence of brain cell-derived EVs in the blood, carrying diverse cargoes, has enabled the development of predictive, diagnostic, prognostic, disease-monitoring and treatment-response biomarkers for various neurological disorders. In this Review, we consider how EV biomarkers can bring us closer to understanding the complex pathogenesis of neurological disorders such as Alzheimer disease, Parkinson disease, stroke, traumatic brain injury, amyotrophic lateral sclerosis and multiple sclerosis. We describe how translational research on EVs might unfold bidirectionally, proceeding from basic to clinical studies but also in the opposite direction, with biomarker findings in the clinic leading to novel hypotheses that can be tested in the laboratory. We demonstrate the potential value of EVs across all stages of the therapeutic development pipeline, from identifying therapeutic targets to the use of EVs as reporters in model systems and biomarkers in clinical research. Finally, we discuss how the cargo and physicochemical properties of naturally occurring and custom-engineered EVs can be leveraged as novel treatments and vehicles for drug delivery, potentially revolutionizing neurotherapeutics.</p>