There cannot have been many circumstances that have challenged the modeller of agricultural markets to the extent that the developments in the EU in recent years have. The enlargement of the EU involving a large number of countries, with important agricultural sectors, many emerging from a volatile transition from centrally planning, raises many issues. Moreover this is occurring at a time of radical reform of the CAP, with the substantial decoupling of payments, an area that has attracted some research but provides little concrete guidance for sector level modellers. In this paper the challenges of each of these developments are outlined and their importance to the sector addressed. Some strategies in dealing with the issues and the impact on the model results are evaluated.