A brief review of the literature, informed by comments from a range of tax specialists, suggests four principal groups of reasons why tax systems do not yield higher revenues in many developing countries: (1) Internal political factors (2) Administrative constraints (3) External political factors: multinational companies and other investors (4) The structure of developing country economies However, the issues involved are complex, and simple and direct answers do not exist. Professor (Emeritus) Richard Bird, chair of the Advisory Group of the International Centre for Tax and Development at the Institute for Development Studies, comments that “My more than 50 years of work on these matters in more than 50 countries continues to provide me more with questions than with answers.”