The "Vote Leave" campaign — made up of some prominent members of the Conservative government, some Labour Party members, and others — has been lobbying hard to sway the vote.
The main reason for leaving the EU, they argue, is that the cost of giving up some sovereignty in order to access the common single market is no longer worth it. Primarily they want to take back complete control of Britain's laws, courts, and migration policies.
That in turn can be linked to their opposition to the idea of "an ever closer union," one of the principles the EU has been acting under since the early 1990s. They worry that if the EU makes more laws that the U.K. has to follow, it could mean more and more of the things a country should be able to do for itself will be transferred to the EU.