Grand challenges stress the importance of multi-disciplinary research, a multi-actor approach in examining the current state of affairs and exploring possible solutions, multi-level governance and policy coordination across geographical boundaries and policy areas, and a policy environment for enabling change both in science and technology and in society. The special nature of grand challenges poses certain needs in evaluation practice: (a) the need for learning at the operational, policy and, especially, system level; and (b) the importance of a wider set of impacts and behavioural change. The examination of the usefulness of evaluations as learning tools thus becomes relevant as does the way current evaluation practices address broader impacts and issues such as behavioural additionality. The suitability of existing evaluation contexts in meeting the specific issues posed by the "grand challenges" orientation is also worth examining. The paper argues that learning at the policy and system levels is largely unaddressed while concepts such as behavioural additionality are still underexploited.