o infrastructure development and consider what infrastructure mix may be desirable in light of the tradeoffs that will have to be made between the various policy objectives. (For example, environmental concerns may lead to attempts to shift traffic from roads to rail or to increase the share of renewable energy sources, but without unduly constraining economic development.) This introduction is organised as follows. In order to put infrastructure development into a proper overall policy perspective, it begins with a brief overview of the economic, social and other benefits that infrastructures have generated in the past and might be expected to provide in the future, before turning to the question of the most important factors likely to shape the development of infrastructures in the coming years. After summarising the findings of projections on future investment requirements from each of the sectoral chapters, it addresses some of the specific interdependencies among the different infrastructure sectors. Finally, it draws out a number of relevant cross-sectoral issues and policy challenges, and then reflects briefly on the way ahead. 1. Benefits from infrastructure - past and future The economic benefits of infrastructure have been the subject of much debate since at least the 1980s, with the discussion focusing on both the direction and magnitude of the effects. While it is possible to establish a link between infrastructure development and economic development, it is difficult to ascertain the direction of the causation: does infrastructure contribute to economic development or vice versa? Moreover, there was considerable scepticism about the initial estimates of the productivity gains stemming from investment in public infrastructure. Over the past few years however - with improved data, new methodological approaches and refinements to models - there has been much less controversy. A review of the more recent literature suggests that public infrastructure has a positive productive effect on the economy, but t