The need to promote increased trade, prevent the spread of HIV/AIDS, and ensure adequate water resources are some of the development challenges generating interest in increased regional and sub-regional cooperation. This evaluation seeks to provide guidance on when it is desirable for the Bank to support activities on a regional level, and assesses the effectiveness of both regional programs and the Bank's role in supporting them. It comes at a time of increasing interest in regional development programs, supported by recent theoretical work on regional public goods and regional trading arrangements, and calls from groups in the international development community for the Bank to provide more support for multi-country programs (both global and regional).