The human gut microbiome is a large community of bacterial microbes, which interact to aid digestion, protect against pathogens and create essential metabolites. Whilst the gut microbiome has been implicated in adverse health outcomes, robust applied epidemiological evidence able to discern causation from correlation does not exist. With my expertise, data resources built in my current position, preliminary results, established collaborations and desire to work within this clinically relevant field, I am setting out to apply robust epidemiological and causal inference methods to human gut microbiome research. The proposed fellowship sits in a specific area for intended future research, in which I wish to establish myself as an academic leader, with the additional aims of further characterising the causal and functional role of the gut microbiome at scale (combining both metagenomics and metabolomics), understanding the range of modifiable causal risk factors that lead to gut microbiomic variation (using MR, randomized controlled trials and genotype-directed recall studies) and identifying therapeutic targets to migrate within an industry or pharmaceutical setting.