The overall microbiome is defined as the gene ensemble specific to bacteria, viruses, archaea, and fungi within an organism. While the gut, the skin, and the oral cavity represent prominent anatomical sites in which the microbiome is present and highly abundant, commensal microorganisms can additionally be found in other organs, including the lungs, prostate, liver, pancreas, breast, and gallbladder. Living bacteria have also been described to be an additional component of the tumor microenvironment in both primary lesions and metastasis. The composition of symbiont bacteria is altered in cancer patients, and in turn, bacteria have been described as having multiple functions in cancer, creating a two-way connection. The bacterial microbiome has been described as having a role during all the phases of the disease, from tumorigenesis to metastasis, also influencing cancer treatments. Thus, the polymorphic microbiome has recently been recognized as a cancer hallmark. This chapter will discuss the milestones connecting the bacterial microbiome to cancer biology.