Most visas (entry clearance) for the UK require an application before you travel to the UK. Some of the most common forms of visa are student visa, work visas (which are now governed by a points-based system), family visas and partner/spouse visas.
The requirements for these visas are becoming increasingly difficult to meet: most require a lot of documentary evidence, large amounts of money, and lengthy probation periods. See Family Migration section.
You should always try and get legal advice before submitting an application. Even if your application is successful, entry clearance officers (the immigration officers who work at ports of entry) can still refuse to let you enter. The right to appeal refusals of visa applications is limited. Since June 2013 there has been no right of appeal against the refusal of a family visit visa application, unless the appeal is on human rights or race discrimination grounds).
European Economic Area (EEA) nationals (broadly speaking, from countries that are in the European Union) can travel to, and stay in, the UK more easily than people from non-European countries.
All visas will be time-limited and so will need to be renewed if you wish to stay longer. It is possible to move between some immigration categories while in the UK, but this is becoming more difficult. If you do not renew your visa, you will be classified as an overstayer. Overstaying is an immigration offence, and UKBA frequently use ‘poor immigration history’ as one reason to refuse release from detention and applications for leave to remain.