An apparatus for capacitive ascertaining and/or monitoring of the fill level of a medium in a container. The apparatus includes at least one probe unit, which is electrically insulated relative to the medium, and at least one electronics unit, which supplies the probe unit with at least one electrical, actuating signal, receives an electrical measurement signal from the probe unit, and evaluates the measurement signal as regards the fill level The actuating signal is an electrical alternating voltage signal. The electronics unit is embodied in such a manner that it supplies the probe unit with the actuating signal in a measuring phase, and supplies the probe unit with a test signal in a test phase, and that it receives a test measurement signal in the test phase from the probe unit, wherein the test signal is selected in such a manner that it has at least one section with an essentially constant voltage value.