This chapter describes the basic principles inside the VEG extension to the ForSAFE model system. It allows changes in ground vegetation to be calculated, an important part of biodiversity. In the VEG model, the basis for modelling ground vegetation dynamics is a competition strength model based on soil chemistry promoting and retarding factors, nutrients, water and light. The strength is used in a competition model to assign ground area to each plant type considered. The ForSAFE-VEG is freely available from the authors and is used for assessing critical loads for acidity and nitrogen in Europe and United Sates, based on biodiversity criteria.