In December 2008, the European Council and the European Parliament agreed on a final compromise for a new European renewable energy directive. One of the most debated issues prior to this compromise was the design of "target flexibility mechanisms", which should allow member states with low or expensive renewable energy potential to partly fulfil their national renewable energy target in other countries. This article traces back the political discussion that has led to the evolution of the different flexibility options. It then evaluates the most prominent flexibility mechanisms against a set of qualitative criteria. It concludes that free or restricted certificate trade based on guarantees of origin (GOs) - as proposed earlier by the European Commission - is not a viable option due to some "knockout" criteria, despite other potential advantages. The mechanisms that have replaced GO trade in the final compromise - joint projects, joint support schemes and statistical transfer between member states -provide less flexibility. but score better against a number of other important criteria. The crucial question for the coming years is how their utilisation can be facilitated. One first step might be that proactive member states define open design issues for implementing the mechanisms. (C) 2009 Elsevier Ltd. All rights reserved.