The "Brexit" vote is the biggest event for financials markets we've seen in a while, one that will be fascinating to watch as votes are counted on Friday (for our time zone).
Financial markets have ignored the Brexit issue for most of this year, but complacency has now been replaced by extreme unease after big swings in the polls.
This has led to a sell-off in UK and European shares, and sharp falls in the pound and the euro. Investors are worried about the consequences of a potential Brexit on the UK, including faltering economic activity and rising unemployment.
The reasons for such anxiety over Europe are more interesting, and are political as well as economic. People believe that a Brexit might give other countries similar ideas, reigniting fears about the future of the Eurozone group and starting a "whose next?" spiral of uncertainty.
While the UK is part of the broader 28-country European Union, they are not a member of the Eurozone, a much more integrated subset. The 19 countries that belong to the latter use the euro as their currency, while the UK has always retained the Pound Sterling.