The mucosal surfaces of the body are characterised by complex, specialised microbial communities, often referred to as the microbiome. However, only much more recently—with the development of technologies allowing exploration of the composition and functionality of these communities—has meaningful research in this area become feasible. Over the past few years, there has been rapid growth in interest in the gut microbiome in particular, and its potential contribution to gastrointestinal and liver disease. This interest has already extended beyond clinicians to pharmaceutical companies, medical regulators and other stakeholders, and is high profile among patients and the lay public in general. Such expansion of knowledge holds the intriguing potential for translation into novel diagnostics and therapeutics; however, being such a nascent field, there remain many uncertainties, unanswered questions and areas of debate.