Vertebrates and insects have evolved complex repertoires of chemosensory receptors to detect and distinguish odours. With a few exceptions, vertebrate chemosensory receptors belong to the family of G protein-coupled receptors that initiate a cascade of cellular signalling events and thereby electrically excite the neuron. Insect receptors, which are structurally and genetically unrelated to vertebrate receptors, are a complex of two distinct molecules that serves both as a receptor for the odorant and as an ion channel that is gated by binding of the odorant. Metabotropic signalling in vertebrates provides a rich panoply of positive and negative regulation, whereas ionotropic signalling in insects enhances processing speed.