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Estimating the revenue cost of tax expenditures and understanding their role in revenue-based fiscal adjustments is one of the most important elements of macroeconomic management, which is likely to take center stage in the debate on how developing countries can mobilize domestic revenues for attaining their sustainable development goals. The quality of policy outcomes hinges on the quality of tax expenditure reporting at the country level, which in turn largely determines their credibility as an analytical tool for policy making. We examine tax expenditures reports of 26 developing and transition countries, and assess whether they reflect good practices, defined according to several key dimensions. Although such reporting has gained momentum in the past decade in developing countries, its quality remains weak in several areas. For example, only about fifty percent of the countries surveyed describe in some detail their benchmark tax system. All countries (except Uruguay) do not include the time frame of tax expenditures (as permanent or temporary). The publication of tax expenditures reports is a legal requirement in only eighteen countries. And many reports are incomplete in their coverage of the tax system and reporting dimensions. Developing countries should improve the quality and comprehensiveness of their reports to gain credibility in developing robust domestic revenue mobilization strategies. Otherwise, these reports are likely to become voluminous and opaque documents, with little benefit to the larger public. Finally, we find that differences in methodology, data and reporting on the cost estimates of tax expenditures across countries are important enough to make cross-country comparisons problematic, perhaps even misleading. JEL Codes: H2, H6.