In the NFI2, several different models were used to assess how far certain forest functions, beyond timber production, were fulfilled. The forest functions included, for example, the protection against natural hazards, the preservation of the biodiversity, and the availability of recreational space. Furthermore, a model for classifying different forest types was used. The employed models contain, as does every model, different types of uncertainties. These uncertainties can be due to underlying assumptions, chosen model structures, values of the model's parameter, and only imprecisely given input values (see as an example the research of uncertainty analysis of models conducted by JOHNSON 1987 and PAHL-WOSTL et al. 1997). The uncertainties in the models themselves have been discussed in the appropriate chapters. This chapter tries to analyze how uncertainties of the input data can affect the outcome of the models.