In recent years, there has been an explosion of literature published in the area of the microbiome on health and disease. The microbiome has even entered the lexicon of the general public. In 2015, the American Museum of Natural History mounted an exhibition titled “The Secret World Inside You” that aimed to teach children and adults about the importance of the microbiome. Fecal transplantation in the treatment of Clostridium difficile infection has been widely reported in the popular media. There are already online businesses that aim to analyze one’s gut microbiota to formulate a personalized diet. But what do we really know about the microbiome? And what role does it play in psychiatry?