‘Fast Facts: The Gut Microbiome’, though pithy and concise, is amazingly comprehensive and takes the reader through all major aspects of the human gut microbiome and microbiome science. The depth of Professor Shanahan’s knowledge of the science of the microbiome is evident throughout but what transforms this volume from a mere recitation of what we know to a truly critical and accessible exposition on what matters is the critical eye of the clinician who must make decisions that will affect a patient’s health.