It has been quite a challenge for econometricians to model economies in transition. There is no textbook at hand to master that task. Economic theory cannot be applied without adaptations to the characteristic change of a whole economic system. Regression analysis, taking into account past economic development only, is of limited use for the econometrician. Having econometric models at hand would be very helpful for an active economic policy to guide the transition process. Various scenarios representing strategies could be simulated in their consequences to the economy. The best alternative in respect to the government's objectives could be chosen. This very situation has born the idea of co-operation between L6dz and Frankfurt in 1990. There are problems of this kind in Poland and in Germany. The German situation is somewhat better than that of Poland as a relatively small centrally planned economy is being united with a substantial social market economy taking over a lot of the burden of the former mismanagement. Thus, it might be possible to share the experience in modelling the united Germany and preparing forecasts with the Polish model builders. In addition, it would be prOfitable for both model establishing teams to link their models in order to improve the forecasting potential. Moreover, the Polish partner has a broad national and international experience in econometric model building which makes co-operation smooth and fruitful. His experience in modelling countries with a centrally planned economy would also help to master the transition problems.