Ritualized and repetitive behaviours are included in the diagnostic criteria for both autism spectrum disorder (ASD) and obsessive-compulsive disorder (OCD). OCD often co-occurs in young people with ASD, yet there is little clinical research on its phenomenology or treatment. The extant research in this area is largely composed of qualitative and single-subject study designs. In this chapter, we synthesize this body of research and provide recommendations for differential and dual diagnostic decision-making and treatment planning. The case of a 16-year-old male with comorbid ASD and OCD is presented, in which we demonstrate how standard cognitive-behavioural therapy (CBT) for OCD may be modified, for instance, to address problems with emotion regulation that hinder progression through an exposure-based therapy.