The personal income tax is widely regarded as the fairest method of taxation yet devised. It is the major element of progression in modern tax systems and permits differentiation of tax burdens on the basis of family responsibilities and other personal circumstances of taxpayers. The yield of the tax expands or contracts more rapidly than personal income, thus imparting built-in flexibility to government revenue systems. The income tax is less burdensome on consumption and more burdensome on personal saving than an equal-yield expenditure tax, but the difference in aggregate terms is probably small for taxes of broad coverage. The effect of the income tax on work and investment incentives is unclear. Although personal income taxation has a long history, some of its major features still present numerous unresolved problems.