Until the late 1960s, obsessive compulsive disorder (OCD) was considered to be a relatively untreatable condition. Over the next several years, many advances were made in the psychological treatment of OCD as clinicians and researchers began to realize the value of behavioural treatments for this population. Isaac Marks and his colleagues played an instrumental role in the development and initial validation of exposure-based treatments for OCD. In addition to his other important research in the area of anxiety, Marks continued to study behavioural treatments for OCD, including the processes and predictors of treatment outcome. More recently, his work has focused on self-administered treatments for OCD and other anxiety disorders, including computerized and telephone-based treatments. This paper reviews research on the behavioural treatment of OCD, with an emphasis on the contributions of Isaac Marks.