The aim of the System Dynamics Model KEYNEO is to model the German economy over a long time period (40 years). Keynesian and neoclassical elements form the base of KEYNEO. In the first step a complex feedback structure was developed to model the main economic variables on an aggregate level. The equations for the supply and the demand side of the economy were defined in the second step. The results of different runs demonstrate that KEYNEO mimics historic data quite good. With the use of optimization tools the parameters could be estimated. The statistical analysis of KEYNEO shows that the results are highly significant. This verification underlines the quality of KEYNEO to model an economy. In addition, the structure of KEYNEO may serve as input for much more sophisticated models.