Accumulating evidence not only supports the functional role of the gut microbiome in cancer development and progression but also its role in defining the efficacy and toxicity of chemotherapeutic agents (5-fluorouracil, CTX, Irinotecan, Oxaliplatin, Gemcitabine, Methotrexate) and immunotherapeutic (anti-PD-L1/ anti-PD-1 and anti-CTLA-4) compounds. This evidence is supported in numerous in-vitro, animal and clinical studies which highlight the importance of microbial mechanisms in defining therapeutic responses.