The term "tax expenditure" refers to departures from the normal tax structure designed to favor a particular industry, activity, or class of persons. Most budget experts view the tax expenditure budget as a useful tool in managing the size and scope of the federal government, but a growing contingent of conservative critics has raised questions about the concept. The paper examines tax expenditure measurement issues, the debate about their relevance and new measures tentatively explored by the Bush Administration's budget, the way tax expenditure estimates are used in the U.S., and how they might be made more useful.