Studies using brain imaging methods have shown that neuronal activity in the orbitofrontal cortex, a brain area thought to promote the ability to control behavior according to likely outcomes or consequences, is altered in drug addicts. These human imaging findings have led to the hypothesis that core features of addiction like compulsive drug use and drug relapse are mediated in part by drug-induced changes in orbitofrontal function. Here, we discuss results from laboratory studies using rats and monkeys on the effect of drug exposure on orbitofrontal-mediated learning tasks and on neuronal structure and activity in orbitofrontal cortex. We also discuss results from studies on the role of the orbitofrontal cortex in drug self-administration and relapse. Our main conclusion is that although there is clear evidence that drug exposure impairs orbitofrontal-dependent learning tasks and alters neuronal activity in orbitofrontal cortex, the precise role these changes play in compulsive drug use and relapse has not yet been established.