The W-4 form, officially known as the Employee's Withholding Certificate, is a document that employees in the United States fill out to inform their employer about how much federal income tax to withhold from their paychecks. This helps ensure that the correct amount of tax is deducted throughout the year, which can prevent underpayment or overpayment when filing annual tax returns. Employees can adjust their withholding by indicating their filing status, number of dependents, and any additional amount they want withheld. Changes in personal circumstances, such as marriage or having a child, may require submitting a new W-4 to reflect updated tax obligations.