Tax expenditures are an instrument frequently used when a government wishes to achieve certain economic and social effects. But because of the increasing number and scope of tax expenditures, their proper use, quality of administration and record-keeping have become a major challenge for the tax authorities and the whole of the government. The article considers and explains very diverse forms of tax expenditure such as reliefs, tax deductions, tax allowances, tax exceptions and special rates of taxation and the ways in which they are defined and calculated. The key problems in the analysis are the absence of a single definition and of methodology for the calculations; these ultimately make it impossible to compare tax expenditures between or among countries.