A method for heating a glow plug for a diesel engine to its desired temperature by supplying power to the glow plug in a controlled fashion. During a certain time interval after termination of a previous glow process, a mathematical model is used to determine the values for the supply of power to the glow plug, which includes the values of the actual thermal state of the glow plug, the time elapsed since the end of the previous glow process and the parameters of the diesel engine relevant for a glow process.