Morphological and functional changes seen in obsessive-compulsive disorder (OCD) are reviewed. OCD and neurology are closely linked: some neurological diseases involving basal ganglia are sometimes associated with OCD symptoms; conversely, OCD patients often present neurological soft signs. Structural neuroimaging has shown reduced caudate nucleus volume among OCD patients. Functional neuroimaging demonstrated repeatedly an activation of the caudate nucleus and orbitofrontal cortex areas which reversed after treatment. Recent studies showed a correlation between induced OCD symptoms and an increase of the blood flow of the right caudate nucleus and the orbitofrontal cortex. A circuit linking the orbital cortex, the striatum and the thalamus is probably involved in OCD symptoms. However, it is not yet possible to find a causal lesion for this trouble.