The question: Colin asks BBC Radio 4's PM programme "If the UK were to leave the EU, how would Gibraltar be affected?"
Reality Check verdict: If the UK leaves the EU, so does Gibraltar. Gibraltar cannot reapply to join the EU on its own without being recognised as a sovereign state.
Gibraltar is a self-governing British overseas territory. However, it depends on the UK for its clout in international relations.
Spain sees Gibraltar as an outdated legacy of the British Empire that "destroys the national unity and territorial integrity of Spain", to quote the latest Spanish government statement on the matter.
In other words, it wants it back.
If the UK leaves the EU, Gibraltar leaves too. It cannot apply to join on its own without being recognised as a sovereign state.
There are fears a vote to leave the EU could mean Gibraltar's border with Spain is closed, cutting it off.
Currently, Gibraltar is protected by EU rules governing the single market, specifically the free movement of people.
Spanish dictator Gen Francisco Franco closed the border in 1969.
It was not reopened until 1985, a decade after his death, as Spain negotiated to join the EU.
Gibraltar sets its own taxes and is a low-tax regime.
Its economy is built on insurance, hosting online gambling sites, financial services, and its port.
Companies can register in Gibraltar and then trade across the whole European Union.
That is made possible by the UK's - and therefore Gibraltar's - position in the single market.
Fabian Picardo, Chief Minister of Gibraltar, has described the EU referendum as a "political meteorite heading right for Gibraltar", with a Brexit "disastrous" for the territory's economy.
About 10,000 people crossed the border to work in Gibraltar every day, he said.
He fears a condition of re-entry to the single market would be joint sovereignty over the territory between the UK and Spain.
The UK government has said its support for Gibraltar is "firm and unconditional".
About 23,000 UK citizens in Gibraltar can vote in the EU referendum. And they are expected to vote to remain, despite the territory's fierce pro-UK traditions and cultural independence.
Incidentally, Spain goes to the polls for a general election on 26 June, three days after the UK's EU referendum.