As of 2014, current Internal Revenue Service withholding tables are found in Section 16 of the IRS Publication 15. Withholding tables change periodically, and it is important that up-to-date tables are used for reports to the IRS.
The IRS provides tables to employers and other administrators who are responsible for withholding tax from employees' paychecks as well as from other sources of income. There are instances where administrators responsible for withholding income tax withhold too little or too much tax. There have also been instances where tax laws have been changed retroactively, so that all taxpayers effectively overpaid when income taxes were withheld at the income source. Withholding too little or too much tax results in a tax refund or payment being necessary when individuals file.