Behavioural experiments (BEs) are amongst the most powerful methods for bringing about change in cognitive therapy. They are a key component of treatment. They are widely used, and yet, to be successful, they require creativ-ity and sophisticated understanding on the part of the therapist. It is therefore surprising that there is remarkably little written about BEs: about their place in cognitive therapy, their value, or about the practicalities of designing and carrying out BEs. It is this gap that the present book seeks to fill. The purpose of this first chapter is to provide some underpinnings for con-ceptualizing the place and role of BEs in cognitive therapy. It is divided into two parts. The first part provides a review of cognitive therapy. It describes the development of the therapy; its standing as a treatment for psychological disorders; its historical roots in behaviour therapy; and its core ideas. The second part focuses on the BE as a key intervention within cognitive therapy. It provides a definition; looks at the historical roots of BEs in the scienti-fic method and in behaviour therapy; examines evidence supporting their effectiveness; and reviews theories which provide some understanding of their impact. Our aim is to provide a historical and conceptual understanding of the value of BEs, while acknowledging that, in the current state of knowledge, there are large gaps to be filled.