Self Assessment is the process by which individuals in the United Kingdom are responsible for calculating and reporting their own tax liability to Her Majesty's Revenue and Customs (HMRC). The self-assessment process applies to individuals who are self-employed, have certain types of income that are not taxed at source, or receive income from abroad.

Under self-assessment, individuals are required to complete and submit a tax return (form SA100) to HMRC each year, outlining their income and gains for the previous tax year, which runs from 6 April to 5 April. Tax returns must be submitted by 31 January following the end of the tax year, and any taxes owed must be paid by the same date.

In the self-assessment process, individuals must declare all income earned, including salary, wages, tips, and investment income. They must also disclose any capital gains made from selling assets, such as property or shares. Additionally, they are eligible to claim reliefs and expenses to reduce their tax liability. Self-assessment can be a complex process and it's important that individuals understand their tax obligations and how to complete their returns accurately to avoid penalties for errors or omissions. It is recommended that individuals seek advice from a tax professional if they are unsure of their obligations or how to complete their tax return.