One advantage of conceptualizing two disorders as related lies in the possibility that a similar treatment is effective for both conditions. The approach taken in this paper is somewhat different, however. The authors have argued that although a particular psychobiologic process may be relevant to OCD and obsessive-compulsive conditions, it may be disrupted indifferent ways in various conditions that fall along a spectrum. If so, differ-ent obsessive-compulsive spectrum disorders may well require different treatments. Thus, although OCD and trichotillomania can be conceptualized in terms of the pathologic release of motor programs, they may require rather different treatment approaches. Furthermore, in the absence of a detailed understanding of the psycho-biology of OCD and related disorders, conceptualizations of obsessive-compulsive spectrum disorders can remain only preliminary in nature. The involvement of cortico-striatal circuitry in OCD suggests that obsessive-compulsive spectrum disorders may be characterized by involvement of these paths. Striatal circuits mediate many different functions (including reward processes), however, and are involved in many disorders. Similarly, the involvement of serotonergic neurotransmitters in OCD suggests that these neurotransmitters are central to defining spectrum disorders.Again, however, serotonin plays a role in many functions (including impulse control) and mediates many different disorders. Much work remains to be done to delineate optimally the obsessive-compulsive spectrum of disorders. Nevertheless, significant progress has been made. Empiric data demonstrate involvement of cortico-striatal circuitry in a number of putative obsessive-compulsive spectrum disorders [43]. Similarly, data demonstrate that a selective response to serotonin-reuptake inhibitors is seen in a range of these different spectrum disorders. As further progress is made in understanding the cognitive-affective neuroscience of OCD and related conditions, constructs about obsessive-compulsive spectrum disorders will become increasingly sharp.