The chapter deals with a section on performance measurement of international organisations in general. It shows that many evaluation models use as a starting point linear causality to measure the performance of international organisations by linking objectives directly to effects. The chapter provides an alternative evaluation model based on complex causality in which the black box of the negotiations is opened. It focuses on what conditions are needed to assess European Union (EU) diplomacy in a multilateral environment. The chapter presents a model that was developed in earlier research to evaluate national diplomatic interventions in multilateral decision-making arenas. The model is adapted to measure the performance of the EU diplomatic interventions in multilateral institutions. The chapter suggests that the ways forward for thinking of EU performance in multilateral institutions are made. The EU explicitly links evaluation to the decision making cycle through ex ante, interim evaluation, ex post evaluations.