SUMMARY Head and neck cancer (HNC) remains a lethal disease despite concerted efforts to improve its diagnosis and treatment. Although smoking, drinking, and HPV infection are closely linked to HNC, emerging evidence also suggests that changes in the human microbiome are associated with HNC. However, little is known about how altered microbiota affect HNC pathogenesis or its treatment. The oral cavity, which is directly relevant to the local microenvironment in HNC, harbors one of the most complex and diverse microbiomes of all human anatomical sub-sites. However, how the oral microbiota shapes the local and systemic environment in HNC pathogenesis is completely unknown. Our long-term goal is to determine the functional role(s) and mechanisms of action of the microbiome in HNC and to translate these findings into novel preventative and therapeutic strategies. In the proposed study, we will extend and deepen previous studies of human microbiota in HNC, focusing on a prevalent form of HNC with high mortality and morbidity, oral squamous cell carcinoma (OSCC). Our Specific Aims are as follows 1) To define OSCC-associated metagenomic and metabolomic signatures in treatment naïve OSCC cases and non- OSCC controls and 2) To determine whether microbiota transfer from human OSCC cases to mouse models promotes tumorigenesis and characterize the underlying molecular mechanisms. Our approach is innovative because it represents a departure from the status quo by utilizing unique experimental mouse models and state- of-art technologies to move beyond observational human studies to delineate the molecular, cellular, and immunological mechanisms induced by the human microbiome in OSCC pathogenesis. The proposed research is significant because it is expected to advance and expand understanding of how the microbiome as a whole, as well as specific microbial taxa, genes, and pathways, impacts OSCC. Ultimately, such knowledge has the potential to be developed into effective therapies for OSCC patients, a pressing need given the significant incidence and poor prognosis of this disease.