1. The Brexit Referendum
May's party, the British Conservative Party, has long been split between those in favour of EU membership and those who want out.
One advantage of membership is it gives seamless access to trade across 27 other countries. The EU is Britain's biggest trading partner.
Opponents have argued the EU took too much political power from London; many want to restrict immigration from other EU member countries as well.
To settle the question, former Prime Minister David Cameron called for a nationwide vote on whether Britain should stay or go. On 23 June 2016, 52 percent of voters said they wanted to leave.