A method for determining the temperature of a sheathed-element glow plug during operation in an internal combustion engine is described, in which a temperature-resistance reference correlation is determined. In order to reduce the tolerance range of a sheathed-element glow plug, after the sheathed-element glow plug has been installed in the internal combustion engine, a calibration step is carried out in which, with the aid of the temperature-resistance reference correlation determined prior to the installation of the sheathed-element glow plug in the internal combustion engine, a temperature-resistance correlation specific to the sheathed-element glow plug is ascertained, from which the temperature of the sheathed-element glow plug during operation of the sheathed-element glow plug in the internal combustion engine is determined.