Diamond has some of the most extreme physical properties of any material, yet its practical use in science or engineering has been limited due its scarcity and expense. With the recent development of techniques for depositing thin films of diamond on a variety of substrate materials, we now have the ability to exploit these superlative properties in many new and exciting applications. In this paper, we shall explain the basic science and technology underlying the chemical vapour deposition of diamond thin films, and show how this is leading to the development of diamond as a 21st century engineering material.