Policies and practice in many sectors affect health. Health impact assessment (HIA) is a way to predict these health impacts, in order to recommend improvements in policies to improve health. There has been debate about appropriate methods for this work. The Scottish Executive funded the Scottish Needs Assessment Programme to conduct two pilot HIAs and from these to develop guidance on HIA. Methods Case study 1 compared three possible future scenarios for developing transport in Edinburgh,

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... on funding levels. It used a literature review, analysis of local data and the knowledge and opinions of key informants. Impacts borne by different population groups were compared using grids. Case study 2 assessed the health impacts of housing investment in a disadvantaged part of Edinburgh, using published literature, focus groups with community groups and interviews with professionals. Results Disadvantaged communities bore more detrimental effects from the low transport investment scenario, in the areas of: accidents; pollution; access to amenities, jobs and social contacts; physical activity; and impacts on community networks. The housing investment had greatest impact on residents' mental health, by reducing overcrowding, noise pollution, stigma and fear of crime. Conclusion Although there is no single 'blueprint' for HIA that will be appropriate for all circumstances, key principles to inform future HIA were defined. HIA should be systematic; involve decision-makers and affected communities; take into account local factors; use evidence and methods appropriate to the impacts identified and the importance and scope of the policy; and make practical recommendations.