The past decade has witnessed an exponential increase in research on exosomes. The capability of exosomes to transfer proteins, DNA, mRNA, as well as non-coding RNAs has made them an attractive focus of research into the pathogenesis of different diseases, including cancer. Increasing evidence suggests that tumor cells release a large sum of exosomes, which may not only influence in local microenvironment, but also can exert systemic effects. In this study, we review the current understanding on this topic, roughly divided into two broad facets of exosomes in cancer: (1) exosomes plays an important role in tumor biological function and mechanism; (2) exosomes can be used as a tumor diagnostic markers, and can monitor the condition changes.