Self-Assessment in the UK is a process managed by HM Revenue and Customs (HMRC) that allows individuals and businesses to report their income and pay the right amount of tax. It applies to those who receive income that isn’t taxed at the source, like income from self-employment, rental income, or other untaxed sources. While most employees have their taxes automatically deducted through PAYE (Pay As You Earn), many individuals must file a Self-Assessment tax return to declare any additional income.