A controller 5 for a high intensity discharge lamp 4 includes a primary ballast for providing a primary current to the lamp 4, and a current injector which in the case shown comprises a control ballast 8 and a switch 9 (e.g. a triac) controlled by a control unit 12. The current injector injects a secondary current into the lamp 4 that is less than the primary current and can be varied by the control unit 12, e.g. by varying the amount of time within an a.c. cycle that the control ballast 8 is switched into the circuit. The controller 5 allows the lamp 4 to be controlled, for example, to operate at a constant power (variable light output) or at a constant light output (variable power).