Tax expenditures are the revenue losses attributable to special provisions in tax laws that reduce taxes for certain sources or uses of income or for certain groups of taxpayers. They are called tax expenditures because many of them effectively serve the same function as direct spending programs despite operating as tax reductions. In this report we briefly discuss some basic tax expenditure concepts: what they are, the different forms they take, why we should be concerned about them, and some criteria for evaluating them.