The paper presents an outline description of the function of each diagnostic system and then considers in more detail their general engineering aspects and some of the development work that has been incurred in meeting specific requirements. The way in which the engineering has differed from previous tokamak diagnostics is discussed illustrating the effect of the Joint European Torus (JET) environmental factors such as radiation, the need for remote handling, and scale. The techniques for interfacing the various diagnostic control systems to the overall machine computerized control system are presented. Finally, three diagnostic systems are described in more detail. From an engineering viewpoint these represent the three basic types: (a) JET design and manufacture, (b) association/JET collaboration (where each has a significant input), and (c) mostly association design and manufacture, but with project monitoring by JET.