Cocaine addiction continues to be an important public health problem with over 1.7 million users in the US alone. Cocaine addiction is characterized by compulsive drug use despite adverse consequences and high rates of relapse during periods of abstinence. Cocaine addiction may be mediated by neuroadaptations in reward-related learning and memory processes in the mesocorticolimbic dopamine system and glutamatergic corticolimbic circuitry. Metabotropic glutamate subtype 5 receptors (mGluR5) likely play essential roles in mediating some of the actions of drugs of abuse. Animal studies have shown that mGluR5 knock-out or blockade reduces self-administration of cocaine and cocaine-induced hyper-locomotion. However, to what extent mGluR5 are involved in the pathophysiology of cocaine addiction in humans is currently unknown, partly due to the lack of suitable methods to reliably quantify mGluR5 in the living human brain.