Intercellular communication plays an important role in the regulation of various cellular events. In particular, cancer cells and the surrounding cells communicate with each other, and this intercellular communication triggers cancer initiation and progression through the secretion of molecules, including growth factors and cytokines. Recent advances in cancer biology have indicated that small membrane vesicles, termed exosomes, also serve as regulatory agents in intercellular communications. Exosomes contain functional cellular components, including proteins and microRNAs (miRNAs), and they transfer these components to recipient cells. This exosome-mediated intercellular communication leads to increased growth, invasion, and metastasis of cancer. Thus, researchers regard exosomes as important cues to understanding the molecular mechanisms of cancer biology. Indeed, several lines of evidence have demonstrated that exosomes can explain multiple aspects of cancer biology. In addition, increasing evidence suggests that exosomes and their specific molecules are also attractive for use as biomarkers and therapeutic targets in cancer. Recent reports showed the efficacy of a novel diagnosis by detecting component molecules of cancer-derived exosomes, including miRNAs and membrane proteins. Furthermore, clinical trials that test the application of exosomes for cancer therapy have already been reported. From these points of view, we will summarize experimental data that support the role of exosomes in cancer progression and the potential of exosomes for use in novel diagnostic and therapeutic approaches for cancer.