In most chemical processes heterogeneously catalyzed gas-phase reactions play an important role. Therefore, the modeling of the fixed bed reactors is increasingly important today. The aim of a model building procedure is to use information already available to predict the behavior of new catalysts, the optimization of process conditions, scale-up, etc. The phenomena occuring in a reactor for heterogeneously catalyzed reactions vary greatly and are very complex, therefore, all models are only approximations with differing degrees of accuracies for the real system. In practice, however, it can be shown that relatively few simple model types can satisfactorily describe a large number of important cases. A priori criteria can aid in the decision when more complex models must be applied.