This chapter focuses on methodological issues, studying in detail what characteristics determine an infrastructure investment project to be pursuing a ‘regional development’ objective, identifying the factors that make them differ from other – more general – projects. It provides some practical hints on the Cost-benefit analysis of air transport projects for regional development. The level of integration between two regions generally stems from three dimensions: geographic, economic, and institutional. Projects affecting several regions, let people assume multinational projects, have an important feature: their effects on the region’s welfare may be greater than the sum of impacts at the individual country level. Deciding whether to build or expand airport capacity or to substantially change any other factor that determines the current equilibria in air transport markets, always poses a difficult challenge for social decision makers.