The nuclear and thermal characteristics of a gas-cooled graphite-moderated reactor, as they affect the control of such plants, are described, and general principles of design are formulated. Interpretation of these principles in terms of main control elements follows, and examples of mechanisms and circuits involved are given. The special operating phases of starting up and shutting down unique to nuclear plants are analysed separately. Throughout the paper emphasis is laid on safety considerations, rendered important by the radioactive nature of the heat source and the exponential nature of nuclear power changes. In addition to the provision of properly designed safety devices, it is urged that the biggest safeguards come from a full understanding of the reactor transient and steady-state characteristics and their careful exploitation in the control-system design.