Due to their size, small extracellular vesicles such as exosomes have been difficult to identify and to quantify. As the roles that exosomes play in intercellular signalling become clearer, so does their potential utility as both diagnostic biomarkers for disease and as therapeutic vectors. Accurate assessment of exosomes, both their number and their cargo, is important for continued advancement in the field of vesicle research. To that end, several technologies, including nanoparticle tracking analysis, have been developed to define the physical characteristics of vesicle preparations and determine their concentration. This chapter describes a method for identifying the size and concentration of a subpopulation of vesicles in biological samples, using nanoparticle tracking analysis. Characterization of distinct exosomes is enabled by specific marker antibodies, coupled to fluorescent quantum dots.