The human digestive system, or gut, is core to our physical, emotional and mental well-being and yet much of its workings remain a mystery to science. In face much of our guts are in fact not human, but rather a vast ecosystem of bacteria, yeasts, fungi, viruses and other micro-organisms collectively weighing up to two kilograms. The connection between the brain and gut is one of the most exciting research areas of biology today, changing the way we manage our mental health and shedding new light on diseases like dementia. This book traces our understanding of the gut, from ancient medicine to modern day, examining the enormous impact this little-understood organ has on our lives and how we might improve the health of our own guts.