In multicellular organisms, effective communication between cells is a crucial part of cellular and tissue homeostasis. This communication mainly involves direct cell-cell contact as well as the secretion of molecules that bind to receptors at the recipient cells. However, a more recently characterized mode of intercellular communication-the release of membrane vesicles known as exosomes-has been the subject of increasing interest and intensive research over the past decade. Following the discovery of the exosome-mediated immune activation, the pathophysiological roles of exosomes have been recognized in different diseases, including cancer. In this review, we describe the biogenesis and main physical characteristics that define exosomes as a specific population of secreted vesicles, with a special focus on their role in oncogenic transformation and cancer progression.