Cognitive Behavioural Therapy (CBT) aims to alleviate distress by helping people to develop more adaptive cognitions and behaviours and is used to treat a variety of common mental health disorders. “The Key Principles of Cognitive Behavioural Therapy” aims to familiarise the reader to this widely used, evidence-based psychotherapeutic method. The article firstly describes the cognitive model of mental illness upon which this type of therapy is based. It further explains key elements of the therapy and some of the main cognitive and behavioural techniques used in CBT interventions. The article includes a brief discussion of the evidence base for CBT and its advantages over other treatments for common mental health disorders. Guidance is provided for when CBT should be recommended, as are links to help readers familiarise themselves with services in their local area. A discussion of whether CBT can be used in primary care concludes the article, along with CBT resources that may be recommended to patients.