Olfactory receptor (OR) genes belong to one of the largest vertebrate gene families, but the number of OR genes can vary greatly among species. Elephant sharks, for instance, have only one functional OR gene, whereas African elephants have ~2,000. The evolutionary processes that gave rise to variation in the number OR genes have been studied extensively with inconclusive results. Analysis of copy number variation in humans indicates that evolution of OR genes can largely be explained by neutral process such as “genomic” drift and random birth-and-death of OR genes. In contrast, broader comparative studies have found that vertebrate OR gene repertoires reflect the properties of ecological niches and anatomical features rather than phylogenetic relationships and that they are therefore adaptive. Systematic evolutionary study of OR genes in vertebrates is limited by: (a) lack of a unified framework to accurately identify OR genes in vertebrate genomes, (b) inconsistent estimates of the number of OR genes for the same species, (c) inclusion of non-OR G-protein coupled receptor sequences (false positives) in analyses, and (d) lack of transparency in reporting genomic co-ordinates for sequences analysed. We have created a framework for systematic evolutionary analysis of vertebrate OR genes that addresses some of these deficiencies. We have also identified ~100,000 OR genes present in all 250 vertebrate species represented in the NCBI database. The analytical framework we have developed and this extensive sample of genes from diverse genomes enable systematic investigation of the mode and tempo of OR gene family evolution and of the evolutionary processes responsible for vertebrate OR repertoires. Using these resources, I will present results of our analysis of OR gene sub-family expansion and contraction across the vertebrates.