**Abstract**

The gastrointestinal (GI) tumour microenvironment is characterised by its unique colonisation with bacteria that are estimated to match the total number of cells in our body. It is becoming increasingly clear that the microbiome and its metabolites are important orchestrators of local and systemic immune responses, anticancer immunity and the host response to cancer therapy. Apart from their role as an energy source, metabolites have been shown to modulate inflammation, immune cell function and cancer cell survival. The polarisation of immune cell subsets by microbial metabolites towards either pro- or antitumorigenic functions strongly affects cancer progression and outcomes. In this chapter, we will discuss the link between microbial metabolites in the GI tumour microenvironment, anticancer immune responses and cancer progression.

**Keywords:** gastrointestinal tumour environment, host immune response, innate immunity, microbial metabolites, metabolism
