To enter the UK you normally need a visa, but there are several exceptions and situations in which a visa is not needed.
EEA nationals (EU nationals + citizens of Iceland, Liechtenstein and Norway), citizens of Switzerland and their family members do not need a visa to enter, reside and work in the UK, as long as the UK has not left the EU. A new scheme will be available for EU citizens and their families to apply to stay in the UK after it leaves the EU. Find out more about the rights of EEA nationals here.
Non-EEA nationals who are family members or ‘extended’ family members of an EEA or Swiss national (excluding UK nationals) enjoy the same rights of residence of a EEA nationals. However, to enter the UK they will need to apply for a EEA Family Permit from outside the UK. Find out more about EEA Family permits here.
Nationals of a number of non-EEA States may also enter without a visa depending on the existing international agreements, although most do.
Check on the UK Home Office website whether you need a visa or not, just by entering your country of nationality: https://www.gov.uk/check-uk-visa.
If you do need a visa, follow our step-by-step guide to a successful UK visa application.