Policy makers need to be mindful of the complexities, unintended side effects and long-term impacts. Evaluation is now taking place in a more complex environment and new demands are being placed on evaluation exercises. Ensuring that these can take account of the overlapping roles and responsibilities of stakeholders, multidisciplinarity, globalisation and more complex funding arrangements will be essential if evaluation is to remain a useful tool for policy makers. In addition, administering assessments can entail considerable costs.