The mobilization of domestic resources through reforms in taxation is essential to ensuring sustainable financing of development. The World Bank Group is engaged in several international initiatives that focus attention on constraints to growth, particularly in low-income economies, where domestic taxes and foreign private and market-related borrowing do not expand enough to compensate for declining flows of official development assistance. This Learning Note reviews existing IEG evaluative evidence on World Bank Group support to tax policy and administration reform over FY2005-15. Over FY2005-15, the vast majority of World Bank support to tax policy and administration reform has been provided through programmatic DPOs. For most of the operations, the tax reform component was a minor part of the operation, predominantly in the 10–14 percent range. In terms of project numbers, the majority of the approved operations were in the Latin America and the Caribbean and Sub-Saharan Africa Regions. IFC Advisory Services in business taxation are usually a small part of investment climate advisory services addressing other issues related to regulatory environment. Tax components in World Bank operations have been designed mostly to enhance revenue to enable fiscal consolidation or create/maintain fiscal space for priority expenditure and/or to improve investment climate or strengthen export competitiveness. With a few exceptions, reviewed DPOs did not specifically address the efficiency and equity of tax systems. The review draws lessons for both the design and implementation of operations and for country programs and World Bank Group strategic engagement in tax reform mobilization.