The reform of the CAP in 1999, with extensive revisions in 2003 (Council regulation 1782/2003), was constructed around three central ideas: decoupling, cross-compliance of payments, and modulation. Decoupling, which is specifically analysed in this article, is intended to eliminate all aid linked in one way or another to production, in favour of aid which is completely independent of the production activity. It is encapsulated in the new 2003 regulation by the introduction of the right to a single farm payment (SFP) proportional to farm area, independent of production activities. These new arrangements have been accompanied by much variation in the application of the reform, and the member states have taken great advantage of the extensive room for manoeuvre inherent in the new regulation. This is revealed in the extreme diversity of choices made, both in the degree of decoupling (total or partial) and in the methods of calculating the SFPs (historical or regionalised references). In order to better understand the reasons for and practical details of the differing national choices, this article presents a synthesis of the results of a comparative study of how the SFPs are managed in four large EU 15 countries (Germany, the UK, Spain and Italy), chosen because of the diversity in their approaches to decoupling and the calculation and trade of the SFPs. Objective economic data specific to each country and national political context are combined to e on the evolution of production systems and xplain the observations, taking into account both the implications of how agricultural holdings, production systems and land are categorised, and the expected effects structures.