The gut microbiota is a significant factor in cancer development and systemic health, affecting tumorigenesis and treatment outcomes. This chapter analyzes the dual function of microbiota in cancer. Some microorganisms can effectively enhance immune responses and suppress tumor growth whereas others may lead to chronic inflammation, produce carcinogenic metabolites, alter immune responses and affect gene expression thereby promoting the development of malignancies such as colorectal, pancreatic, liver, breast and lung cancers. The gut microbiota affects the effectiveness and toxicity of cancer treatments including chemotherapy and immunotherapy, positioning it as both a biomarker and a therapeutic target. Innovative strategies such as fecal microbiota transplantation, engineered bacteria for targeted tumor therapy, precision probiotics, synbiotics and microbiome diagnostics are being increasingly investigated to enhance treatment outcomes and minimize adverse effects. Microbial metabolites including short chain fatty acids and indoles highlight the potential for microbiota to modulate cancer pathways. Despite significant progress, challenges remain including inadequate large scale clinical validation, limited understanding of mechanisms, substantial inter individual variability and a lack of standardized methodologies. Future directions emphasize precision microbiome engineering, artificial intelligence-driven predictive tools and multiomics strategies to develop personalized microbiome guided interventions. This chapter underscores the transformative potential of microbiota-based strategies in cancer prevention and therapy highlighting the necessity for additional mechanistic research, standardized clinical frameworks and interdisciplinary collaboration to bridge current gaps in science and translation.