If applied to real fieldsituations simulation models have to cope with the intrinsicheterogeneity of field soils and the variability of boundary conditions.Complex approaches often fail here because the spatial distribution ofparameter functions can in general not be measured (or otherwiseassessed) with sufficient accuracy. The boundary conditions are oftenpartly unknown. By way of several case studies, I will illustratetypical sources of variability encountered in applications and presentsimplified approaches that were successful in modeling the fate ofenvironmental chemicals in field soils at different scales.