The role of extracellular vesicles (EV) in carcinogenesis has become the focus of much research. These microscopic messengers have been found to regulate immune system function, particularly in tumorigenesis, as well as conditioning future metastatic sites for the attachment and growth of tumor tissue. Through an interaction with a range of host tissues, EVs are able to generate a pro-tumor environment that is essential for tumorigenesis. These small nanovesicles are an ideal candidate for a non-invasive indicator of pathogenesis and/or disease progression as they can display individualized nucleic acid, protein, and lipid expression profiles that are often reflective of disease state, and can be easily detected in bodily fluids, even after extended cryo-storage. Furthermore, the ability of EVs to securely transport signaling molecules and localize to distant tissues suggests these particles may greatly improve the delivery of therapeutic treatments, particularly in cancer. In this chapter, we discuss the role of EV in the identification of new diagnostic and prognostic cancer biomarkers, as well as the development of novel EV-based cancer therapies.