A method for controlling the surface temperature of a glow plug. A heating current flowing through the glow plug and a voltage applied to the glow plug are measured, a temperature-dependent control variable is calculated from measured values of the heating current and the voltage using a first calculation rule, a target value of the control variable is calculated from a target temperature using a second calculation rule, the control variable is compared with the target value and, to minimize any deviation, the duty cycle of the pulse-width modulation is changed in accordance with this deviation. A value other than electrical resistance dependent on current and voltage is used as the control variable. The calculation rule in accordance with which this variable is calculated from measured values of current and voltage is provided to carry out the method and is established in each case for a series of glow plugs.