An essential characteristic of health impact assessment (HIA) is that it seeks to predict the future consequences of possible decisions for health. These predictions have to be valid, but as yet it is unclear how validity should be defined in HIA.To examine the philosophical basis for predictions and the relevance of different forms of validity to HIA.HIA is valid if formal validity, plausibility and predictive validity are in order. Both formal validity and plausibility can usually be established, but establishing predictive validity implies outcome evaluation of HIA. This is seldom feasible owing to long time lags, migration, measurement problems, a lack of data and sensitive indicators, and the fact that predictions may influence subsequent events. Predictive validity most often is not attainable in HIA and we have to make do with formal validity and plausibility. However, in political science, this is by no means exceptional.