Definition: A clinical psychologist is a mental health professional with highly specialized training in the diagnosis and psychological treatment of mental illness, including obsessive compulsive disorder (OCD). Clinical psychologists do not prescribe medications to treat mental illness; rather, they use psychological techniques, such as cognitive behavior therapy (CBT) and psychodynamic therapy. Clinical psychologists must usually complete a Ph.D. in clinical psychology before being able to practice; however, in some states and provinces, a Master’s degree is sufficient. In most states and provinces, the professional activities of clinical psychologists are regulated by a licensing board and/or professional college. In addition to delivery of psychotherapy, psychologists undertake a variety of activities, including psychological testing, research and teaching.