In this review paper by Nienke Koopman, Drosos Katsavelis, Stanley Brul and collaborators from the AMC the multifaceted role of serotonin within the gut is discussed. Besides its well-known function as neurotransmitter, serotonin, also called 5-HT, plays a role in intestinal homeostasis by interacting with immune cells, epithelial cells and even the microbiota. In this review we also elucidate on the role 5-HT plays in inflammatory bowel disease, a disease in which gut homeostasis is disturbed, and the potential of 5-HT targeting therapies.