A self-assessment tax return is a system used by the UK government to collect income tax from individuals and businesses. Taxpayers are responsible for reporting their income, expenses, and any tax owed to the HM Revenue and Customs (HMRC) by completing a tax return form. This process allows individuals to calculate their own tax liability based on their financial situation. Self-assessment is typically required for self-employed individuals, company directors, and those with additional income sources. Taxpayers must submit their returns annually, usually by January 31st for the previous tax year. Failing to file on time can result in penalties and interest charges.