Is obsessive-compulsive disorder (OCD) best categorized as an anxiety disorder? This question has been raised previously, but advances in the psychobiology of OCD and the anxiety disorders, and preparations for Diagnostic and Statistical Manual of Mental Disorders-V and International Classification of Diseases-11, make reconsideration timely. The debate in turn raises the more general issue of how best to address any question of the form "is disorder x in category or spectrum y?" Such questions are related to a number of key debates in philosophy of science and language and have also increasingly been addressed by the cognitive-affective neuroscience of categorization. Here, we review this background debate and use OCD as a relevant exemplar.