The hormones glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) are secreted postprandially from intestinal K- and L-cells, respectively. As incretins, these hormones stimulate insulin secretion from the pancreatic beta-cell, and have independently been implicated in the control of food intake and lipid metabolism. Whilst the enteroendocrine cells producing GIP and GLP-1 are therefore attractive targets for the treatment of diabetes and obesity, our understanding of their physiology is fairly limited. The mechanisms employed to sense the arrival of carbohydrate, fat and protein in the gut lumen have been investigated using organ perfusion techniques, primary epithelial cultures and cell line models. The recent development of mice with fluorescently labeled GIP or GLP-1-expressing cells is now enabling the use of single cell techniques to investigate stimulus-secretion coupling mechanisms. This review will focus on the current knowledge of the molecular machinery underlying nutrient sensing within K- and L-cells.