Simulation models can be efficient tools for several tasks especially in planning. In the USA, the HELP model was developed to simulate the water balance of landfill liner systems, especially of cover systems. HELP version 3 was examined in a German climate in an extensive validation study, and a German adaptation was developed. The validation study consists of three parts: a methodological part (meaning, aims and procedure of validation), a theoretical part (comparison of the modelling approach with the state of science, comparison of documentation, literature and source code), and a practical part (sensitivity analyses and operational validation). In the operational validation, measured and simulated discharges and actual evapotranspiration data of test fields on the landfill Hamburg-Georgswerder from 1988 to 1995 were compared. Limitations, weak points, and errors of the HELP model are shown. The HELP model is a suitable tool for experts in hydrology; but good knowledge of the model and its behaviour and critical review of the simulation results are essential.