Obsessive–compulsive disorder (OCD) is characterized by repetitive thoughts and behaviors that are experienced as unwanted. Structural magnetic resonance imaging (MRI) studies, using different techniques such as manual tracing, voxel-based morphometry (VBM), cortical thickness analysis, and diffusion tensor imaging (DTI), investigating brain abnormalities in OCD patients have been numerous. These studies have implicated the cortico-striato-thalamo-cortical (CSTC) circuit in the pathophysiology of the disorder. However, results have not always been consistent. Variability in study results may partially be explained by small sample sizes, clinical heterogeneity between patient samples, and methodological differences between studies. In this chapter, we review the most consistent findings on morphological brain alterations in patients with OCD, and we discuss the relationship within the implicated networks. The reviewed literature shows that the pathophysiology of OCD cannot be explained by alterations in function and structure of the classical CSTC regions exclusively and emphasizes the importance of other fronto-limbic and frontoparietal areas as well as the cerebellum. Moreover, these findings support the notion that the brain alterations found in OCD patients are represented at the network level rather than discrete brain regions. The widespread abnormalities across several different regions and circuits, and their interconnectivity, fit with the complex phenomenology of OCD, which includes different emotional, cognitive, and behavioral domains. A life span perspective on the structural brain abnormalities in OCD is warranted since variation in developmental stage, symptom profile, and disease stage seems to underlie variation in structural abnormalities.