The tokamak (a doughnut-shaped vacuum chamber surrounded by magnetic coils) is the principal tool in controlled fusion research. This book acts as an introduction to the subject and a basic reference for theory, definitions, equations, and experimental results. Since the first introductory account of tokamaks in 1987, when the tokamak had become the predominant device in the attempt to achieve a useful power source from thermonuclear fusion, and the developments and advances in the subject covered in the second edition in 1997, following substantial research on large tokamaks (the long awaited achievement of significant amounts of fusion power and the problems involved in designing and building a tokamak reactor), the emphasis has been on preparing the ground for an experimental reactor. In addition, there have been further significant advances in understanding plasma behaviour, such as the wider experience of internal transport barriers, the appreciation of the role of tearing models driven by neoclassical effects and insights from turbulence simulations. The fourth edition has been completely revised, bringing all aspects up-to-date and describing the development of tokamaks to the point of producing significant fusion power. It also now addresses the issues relating to the design and future operation of the international tokamak ITER.