Investment in the NHS has increased significantly under the Blair government. Spending will soon reach the EU average, but when we catch up with our European neighbours, what then? Assuming that pressures to spend more will continue, but that marginal health returns on extra investment are likely to diminish, this paper asks the question : how much is enough? In particular, can a limit on health spending be defined, and if so, how? And what evidence is required to inform what is ultimately a political decision? [Introduction]