The Electoral Commission has announced the official lead groups which will campaign for the UK to remain or leave the EU in the upcoming referendum.
Here's a look at the key players for both sides.
Britain Stronger In Europe:
David Cameron: The prime minister is campaigning for the UK to remain part of the EU. He recently wrote: "We can choose economic security, not an unnecessary leap in the dark. We can choose to be stronger, safer and better off."
Former Labour Home Secretary Alan Johnson
Lord Rose: The former executive chairman of Marks & Spencer is the chairman of the remain campaign.
Will Straw: Son of former Labour home secretary Jack Straw, he is the executive director of the campaign.
Michael Gove: The Cabinet minister is the co-convener of the campaign's committee - one of around 130 Tory MPs backing Brexit.
Gisela Stuart: The Labour MP is the other co-convener of the campaign's committee - one of only a handful of Labour MPs campaigning for Brexit.
Matthew Elliot - chief executive of campaign
Iain Duncan Smith