This issue of Translational Cancer Research addresses the current state of knowledge regarding exosomes in body fluids and cancer. Exosomes are produced in increased numbers in cancer compared to normal cells. Exosomes are made by almost all cells, with the possible exception of red blood cells and are secreted into the tumor or normal cell adjacent body fluid. Cancer cell derived exosomes influence tumor growth, metastasis, evasion of the immune system and resistance to therapy.