Opiates interact with cell surface receptors on neurons involved in the transmission of information along neural pathways that are related to behaviours essential for the life of the self and of the species. Opiates are provided with powerful and multifaceted rewarding properties that are fundamental for the acquisition, maintenance and relapse of opiate addiction. Gaetano Di Chiara and Alan North argue that both dopaminergic and non-dopaminergic systems are involved in opiate reward, and that opiate addiction results from adaptive and learning processes involving both positive reinforcing mechanisms related to the rewarding properties of opiates and negative reinforcing mechanisms related to the aversive properties of withdrawal in dependent subjects.