The basic philosophy of a sales tax is that it should be a general levy on consumption expenditures. The early state sales taxes provided general coverage of commodity purchases, but they included many nonconsumption transactions, and they excluded services. There has been a limited trend over the years to add some services. But the major trend has been toward erosion of the base. Part of the trend, and particularly the exemption of industrial and farm machinery and equipment, can be defended on the grounds that these changes bring the taxes more in line with the basic philosophy of the sales tax. But the major form of erosion has been the steady increases in the exemption of consumption purchases, particularly food (once rare, now found in a majority of states), medicines, household fuel, other items, and sales to various nonprofit organizations. Most of these exemptions have been pushed by groups seeking to lessen regressivity of the tax -- but most do so in a fashion objectionable in many respects, compared to alternatives. Virtually all exemptions complicate the operation of the taxes. Legislatures have not been at all careful in establishing the exact form of exemptions in such a fashion as to minimize operational difficulties. There are two major lessons from experience: one exemption breeds another, and exemptions, once provided, are very difficult to remove. Despite the erosion, most of the taxes do remain relatively broad in scope.