**2.1 What is an extracellular vesicle?**

For the past decade the classification of EVs has been based on size, ranging from exosomes that are 30–100 nm, microvesicles (MVs) that are 100–1000 nm, and apoptotic bodies that are 0.1–5 μm [1]. Exosomes are the smallest EV and appear to originate within the lumen of multivesicular bodies [1]. Oncosomes, a subtype of MVs, are released by budding from malignant cells [1]. As cells undergo apoptosis, they release apoptotic bodies [1]. One factor that limits this classification system is that some EVs that function as oncosomes are larger than the typical 100–1000 nm and can be as large as 1–10 μm [1]. When trying to isolate and study EVs, it became apparent that size did not adequately capture the breadth of heterogeneity among EVs with their varied functions and content. In 2019 the International Society of Extracellular Vesicles published a recommendation for the use of the term *extracellular vesicle* to encompass all types of EVs while still including a subclassification system that incorporated size [11]. Therefore, in this chapter the term *extracellular vesicle* will be used. Eventually, the optimal method of classification for EVs will be based on the specific phenotype and content of an EV that would better describe its origin and function. However, current testing methods and understanding of this topic need to be further studied.
