This paper presents the analysis, design, and experimentation of a closed-loop metal halide (MH) lamp ballast, which supplies the lamp with a low-frequency square waveform. First, a discussion related to the lamp parameter that should be regulated (power or current) is presented, since this issue has not been well addressed in the previous literature. Experiments demonstrated that lamp current can be chosen to be kept constant during lamp life, because it can attenuate the effect of lumen depreciation with lamp ageing, and, on the other hand, the lamp power does not reach overrated values during lamp life. Second, in order to analyze the open-loop characteristic and design a suitable controller for the closed-loop operation, a dynamic model of the lamp-ballast system is developed. Therefore, a phase-lag controller is proposed to minimize the steady-state error and provide a phase margin of about 80^° (fully stable system). Experimental results using a flyback-based ballast supplying 35-W MH lamps with 100, 2500, and 5000 burning hours are presented to validate the theoretical analysis. It is demonstrated that the lamp current is kept constant at rated value (0.42 A) within the entire input voltage range. The time response of the complete closed-loop system is about 5 ms.