The U.K. and the European Union are finally about to start discussing the nitty-gritty of how the former will leave the bloc – an unprecedented moment in European history.

British and European officials are gathering in Brussels Monday. On the table will be decisions such as how much the U.K. needs to pay the EU before it leaves.

CNBC takes a look at the key players in the negotiations who will determine how the U.K. is going to leave the Union, and what its future relationship with the trading bloc will be.