In recent years, there has been a growing interest in the gut microbiota and its role in human health and disease. This important role is influenced by the composition of microbiota. A normal microbiota composition plays an important role in maintaining human health, while an alteration in microbiota composition and their metabolism can lead to several diseases including cancer. Recent studies have focused on the role of gut microbiota in cancer development, progression, metastasis, and response to therapy. Certain microbial communities can promote cancer cell migration, invasion, and formation of premetastatic niche, which are crucial in metastasis. On the other side, some metabolites secreted by the microbiota were found to inhibit metastasis. Gut microbiota can have an impact on the efficacy and side effects of anticancer drugs including chemotherapy, target therapy, and immunotherapy. For instance, manipulation of gut microbiota could modulate metastasis and therapy-induced side effects and may be used as an adjuvant therapy to enhance therapeutic outcomes. In this chapter, we explore the complex role of gut microbiota in cancer, cancer metastasis, and their effects on specific anticancer drugs. Potential microbiota-modulating strategies to improve cancer treatment were also discussed.