The wide gap between tax theory and tax practice may be traced back, among other things, to the focus in tax theory on the level and optimal structure of tax rates, assuming that tax bases are consistently defined, precisely measurable and readily and uniformly assessable. This approach overlooks the variances between tax design, tax law, tax impact, tax incidence and tax perception. The effects of taxes on efficiency and equity depend not just on the taxâ€rate schedule adopted but also on differences in tax treatments resulting from the definition, measurement and assessment of tax bases. The Italian experience in the field of income taxation shows the extent to which the definition, measurement and assessment of tax bases matter. Many problems associated with defining, measuring and assessing tax bases, which the personal nature and high progressivity of income tax had contributed to highlighting and accentuating, remain still unresolved and require further research of a better understanding of the rationale, causes and effects of many differentiated tax treatments.