The paper discusses the basis of a life specification for thermionic valves, similar to those in force for electric lamps, and indicates the assistance which a life-test equipment and data afford in preparing such specifications. The general characteristics of thermionic valves are then described, in relation to the problems involved in designing such an installation. Solutions of these problems are then discussed more fully, and a large installation for testing the life of valves is described. Typical examples of the results obtained are given.