Exosomes, extracellular vesicles featuring phospholipid bilayers, play essential roles in cell–cell communication; tumor microenvironment (TME) remodeling (e.g., through cancer-associated fibroblast synthesis and tumor angiogenesis stimulation); and the migration, invasion, and metastasis of cancer cells. In this chapter, the role of exosomes in tumor metastasis is explained. Tumor-derived exosomes, which contain cancer-related proteins and nucleic acids, facilitate tumor growth and metastasis through various means. For example, they mediate and can withstand the antitumor immune response. They allow colorectal cancer cells to escape the immune response by contributing to the delivery of long noncoding RNA from those cells to natural killer cells. In lung cancer, tumor-derived exosomes can initiate cell proliferation, angiogenesis, the epithelial–mesenchymal transformation, and tumor metastasis-associated signaling pathways. Tumor-derived exosomes can also activate endoplasmic reticulum stress pathways in endothelial cells, thereby inducing the loss of vascular integrity and facilitating metastasis. Exosomes released from primary tumors move to recipient cells, where they trigger direct or indirect cell regulation mechanisms that promote tumor progression through further TME alterations. However, exosomes can also inhibit tumor growth by upregulating the expression of surface antigens to trigger immune recognition. The exosome-based delivery of anticancer agents confers various benefits. Using exosomes as a vehicle for paclitaxel enhances the pharmacokinetic properties, bioavailability, and antitumor effectiveness of the drug. With their high biocompatibility and their precise immunoregulatory and tumor-targeting abilities, as well as their ability to inhibit tumor angiogenesis and metastasis, exosomes have high potential for application to cancer treatment development.