Chemoreception is the most important sense for the survival and reproduction of most animal species. In insects, the principal proteins involved in the recognition of chemical cues comprise moderately sized, multigene families. These families include (i) odorant-binding (OBPs) and chemosensory (CSPs) proteins, which are involved in peripheral olfactory processing; (ii) the chemoreceptor super-family, formed by the olfactory (OR) and gustatory (GR) receptors; and (iii) the ionotropic receptors (IR), a variant class of ionotropic glutamate receptors (iGluR). Recent comparative genomic analyses of fully sequenced arthropod genomes support the birth-and-death model as the major evolutionary mechanism in determining the chemosensory repertoire size, and provide evidence of adaptive changes fostered by ecological shifts that might influence the current size of chemosensory families. These studies also advocate for the origin of olfactory gene families with the evolution of terrestriality in insects.