This chapter provides a practical guide to assessing obsessive-compulsive disorder (OCD) that is both informative to the inexperienced clinician and addresses questions raised by the experienced clinician. It will summarize the diagnostic criteria for OCD, including advice on making a differential diagnosis when presented with symptoms that are associated with other disorders; for example, differentiating OCD from psychosis, generalized anxiety disorder, or health anxiety. It will guide the reader through the process of conducting a thorough assessment of the patient’s presenting problems, including OCD and any comorbid problems. The chapter will offer guidance on how to engage the person with OCD and promote trust. There is clear guidance on risk assessment, differentiating between primary risk factors (which clinicians are typically very good at assessing), and secondary risk factors (which may easily be overlooked). Advice on structuring an assessment and on appropriate assessment tools is provided.