In a 1956 presentation at the American Psychological Association's conference in Chicago, Albert Ellis argued that illogical ideas and philosophies were the basic causes of emotional disturbance. In 1961, Ellis called his approach Rational-Emotive Therapy (RET) to emphasize the emotive aspects of his treatment. In 1993 he rebranded his system of psychotherapy as Rational Emotive Behaviour Therapy (REBT) to highlight his longstanding use of behavioural interventions. This chapter introduces the newcomer to the core theoretical principles and therapeutic strategies of REBT. It starts with the ABC model of emotion that Ellis popularized from the outset when he developed his approach to cognitive behavioural therapy (CBT). The chapter then shows how REBT distinguishes between unhealthy and healthy negative emotions. REBT has been applied to emotional disorders involving depression, anxiety and anger as well as behavioural disorders like substance abuse and a lack of self-discipline.