Abstract The forthcoming instrument on European contract law, be it in the shape of an optional code for cross-border contracts or as an official toolbox for the European legislator, is likely to have a spill-over effect on private law adjudication in Europe. Judges will have no great difficulty in finding model rules and definitions that might come in handy when dealing with gaps and ambiguities in European private law. However, the question is whether such a role as a toolbox for judges would be legitimate. I discuss three types of possible legitimation strategies: traditional methods of legal interpretation, the new European methods and merely political legitimation. It will often depend on the circumstances of the case at hand and the characteristics of the particular model rule or definition that is being borrowed what mode of legitimation will prove to be more convincing. However, generally speaking legitimation in terms of the 'general principles of civil law' that the CJEU has recently been developing seems a particularly promising strategy. On the other hand, it seems unlikely that European courts could come under a duty, following from the principle of sincere cooperation, to use the instrument as a toolbox.