Measurements with a single Langmuir probe are in principle a simple electrostatic problem. The probe, the plasma, the power supply and the vacuum vessel form a closed electric circuit. If all resistivities except the sheath resistance in front of the probe can be neglected the measured nonlinear I – V characteristic allows for an evaluation of density and temperature of the plasma. For a complete analysis we have at least to discuss the influence of all other possible resistivities and therefore we must know the exact way, where and how the current path inside the plasma is closed. Dedicated measurements for the flush mounted target plate probes at ASDEX Upgrade as well as theoretical models show that the sheath resistance at the return area has to be accounted for whereas the parallel Spitzer resistivity can be neglected under most circumstances.