The incretin hormones glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are released from enteroendocrine cells in the intestinal epithelium in response to nutrient ingestion. The actions of GLP-1 and GIP - not only on local gut physiology but also on glucose homeostasis, appetite control and fat metabolism - have made these hormones an attractive area for drug discovery programmes. The potential range of strategies to target the secretion of these hormones therapeutically has been limited by an incomplete understanding of the mechanisms underlying their release. The use of organ and whole-animal perfusion techniques, cell line models and primary L- and K-cells has led to the identification of a variety of pathways involved in the sensing of carbohydrate, fat and protein in the gut lumen. This review focuses on our current understanding of these signalling mechanisms that might underlie nutrient responsiveness of L- and K-cells.