Since 1993 we have been using a program developed by the BGR for the automatic inversion of frequency-domain helicopter electromagnetic (HEM) data into the parameters of a layered half-space. This Marquardt-type inversion procedure uses sounding curves for generating suitable starting models. To better understand the results, model calculations have been carried out using 1-D and 3-D resistivity models to generate synthetic data which were inverted using homogeneous or layered half-space models. Inversion results demonstrate that reliable models can be obtained if sufficient and accurate data are available. In particular, we have shown that the 1-D inversion program provides very useful results even in the presence of lateral changes in resistivity. Several examples of inversion results based on theoretical data and survey data are discussed.