Taxable income refers to the base upon which an income tax system imposes tax. Generally, it includes some or all items of income and is reduced by expenses and other deductions. The amounts included as income, expenses, and other deductions vary by country or system. Many systems provide that some types of income are not taxable and some expenditures not deductible in computing tax. Some systems base tax on taxable income of the current period, and some on prior periods.