The peptide cholecystokinin (CCK) is widely distributed throughout the mammalian brain where it functions as a neurotransmitter and neuromodulator. CCK is colocalized with other neurotransmitters at a variety of brain sites. CCK receptor subtypes have been identified and their distributions mapped in the brain and the periphery. Exogenous CCK exerts a variety of effects depending on the site of administration, and many of these effects mimic actions of the endogenous peptide. Progress in understanding of the potential role of CCK in brain and behavior has focused on four main areas: CCK/dopamine interactions, CCK in anxiety and panic states, CCK in opioid nociception, and CCK in satiety. CCK's actions in modulating the activity of other neurotransmitters systems or in affecting behavior may serve as the bases for future therapeutic interventions.