Olfaction is an important means by which insects perceive external environmental cues, and odorant receptors are key proteins for detecting and recognizing odor molecules. The Hymenoptera is comprised of social, parasitic and predatory species, and have a correspondingly diverse range of odorant receptors. The number of hymenopteran odorant receptor genes differs significantly among species. Generally, eusocial insects have more odorant receptor genes, but a few solitary species also have relatively high numbers of these genes. There is currently no universally accepted explanation for this. As in other insect orders, most hymenopteran odorant receptor genes are highly expressed in adult antennae. In addition, the expression of odorant receptor genes differs among different grades and subgrades of social insects, and is affected by host insects and plants in parasitic insects. Some receptors respond to plant volatiles, others to compounds involved in social behavior or host insect chemicals. Research on the Hymenoptera has increased understanding of insect odorant receptors. However, the large number of hymenopteran species and odorant receptors, as well as the complexity of insect olfaction, means that further research is needed to fully understand the role of these genes.