This review examines the gut microbiome's role in cancer development, focusing on how it influences tumor progression, inflammation, and immune responses. Specific microbes, like Enterococcus faecalis and Escherichia coli, promote chronic inflammation, oxidative stress, and DNA damage, activating oncogenic pathways such as NF-κB. The microbiome also affects the efficacy of immunotherapies, including immune checkpoint inhibitors. While variability in microbiome profiles poses challenges to clinical integration, the therapeutic potential of microbiome-based cancer treatments is promising. Understanding these mechanisms could lead to personalized interventions that improve treatment outcomes and enhance immunotherapy effectiveness in cancer patients.