A group of G Protein Coupled Receptors (GCPR's) originally discovered and named for their measurable affinities to various opiates, widely expressed throughout the nervous system, and involved in processes such as nociception, stress, mood and reward.

Classically, three different types were identified and cloned (Delta, Kappa and Mu), although another with no affinity for opioids was later identified through homology (Nociception receptor; initially named ORL1). They all associate with inhibitory G proteins and therefore tend to reduce neuronal activity.

They have been seen to homo- and heterodimerize, but it is unclear if this is physiologically relevant.