Simple SummaryClinical oncology urgently needs more specific and helpful new biomarkers to improve the diagnosis and prognosis of cancer. Research of the last decade proposes extracellular vesicles, particularly exosomes, as a natural source of new biomarkers; since tumors massively release them, they circulate through the body and can be detected and characterized in plasma samples of tumor patients. After a decade of up-and-coming pre-clinical research, the results of the few clinical studies have provided some exciting data supporting the use of exosomes, at least in the follow-up of tumor patients. However, the most convincing data have taught us that, on the one hand, circulating exosomes deliver known tumor markers, such as PSA; on the other hand, the exosome plasmatic levels in tumor patients consistently exceed those of normal controls. This information will be extremely useful in the clinical management of tumor patients.AbstractExosomes are extracellular vesicles (EVs) of nanometric size studied for their role in tumor pathogenesis and progression and as a new source of tumor biomarkers. The clinical studies have provided encouraging but probably unexpected results, including the exosome plasmatic levels’ clinical relevance and well-known biomarkers’ overexpression on the circulating EVs. The technical approach to obtaining EVs includes methods to physically purify EVs and characterize EVs, such as Nanosight Tracking Analysis (NTA), immunocapture-based ELISA, and nano-scale flow cytometry. Based on the above approaches, some clinical investigations have been performed on patients with different tumors, providing exciting and promising results. Here we emphasize data showing that exosome plasmatic levels are consistently higher in tumor patients than in controls and that plasmatic exosomes express well-known tumor markers (e.g., PSA and CEA), proteins with enzymatic activity, and nucleic acids. However, we also know that tumor microenvironment acidity is a key factor in influencing both the amount and the characteristics of the exosome released by tumor cells. In fact, acidity significantly increases exosome release by tumor cells, which correlates with the number of exosomes that circulate through the body of a tumor patient.