**1. Introduction**

Extracellular vesicles (EVs) are membrane-bound particles shed into the extracellular environ‐ ment by many types of cell under different circumstances ranging from normal physiological conditions to pathological conditions like cancer. There are several ways of classifying EVs, including size and mode of biogenesis. Some authors use the designation EVs interchangea‐ bly with other terms like exosomes and microvesicles (MVs). This has led to some confusion andinconsistencyastotheparticlesactuallybeingstudied.Therefore,wehaveincludedasection on the various isolation procedures to emphasise the importance of standardization. Indeed, in many studies, there is no or unconvincing characterization of preparations. In this chapter, we will use EVs as a broad term to encompass three categories: exosomes, microvesicles and apoptotic bodies [1]. Exosomes (30–100 nm diameter) are formed in multi-vesicular bodies (MVB) [2] and released upon MVB exocytosis [3]. They carry several kinds of cargo, depend‐ ing on the surrounding physiological conditions prevailing at the time of their formation and this will determine their effect upon recipient cells. MVs (100 nm–1 μm) are produced by the outward blebbing and fission of the plasma membrane and appear to have possibly more selectively sorted cargo. MVs express surface receptors that differ depending on the mem‐ brane composition of the donor cell [2]. Apoptotic bodies (1–5 μm) are usually released by tumour cells undergoing apoptosis, are packaged indiscriminately and are often fragmented nuclei and cytoplasmic organelles [3, 4] **Figure 1**.

**Figure 1.** Schematic illustrating the relative sizes of the different classes of EVs (Adapted with permission from Ref. [3]).

When first discovered, the release of EVs from cells was thought to be a mechanism for removal of waste and harmful substances from the cell. Nowadays, they are viewed as mediators of intercellular communication through the transfer of biologically active molecules from donor to recipient cells where they can modulate the phenotype and function of those recipient cells [1]. EVs can interact through their surface proteins with receptors on the target cell, triggering intracellular pathways, or by direct membrane fusion or endocytosis thereby releasing their cargo into the recipient cell [5]. Furthermore, EVs could transfer paracrine oncogenic features locally between different cells and endocrine signals to distal cells of any type through body fluids, usually blood [6].

The importance of exosomes may include antigen presentation and immune-stimulatory and inhibitory functions. Several key roles of MVs have been suggested to include contribution to the proinvasive character of tumours, induction of oncogenic cellular transformation, procoa gulant activity and fetomaternal communication [3].
