Boris Johnson and Jeremy Hunt face a gruelling month of campaigning before one of them is crowned the next Prime Minister.
They are tasked with convincing the majority of Conservative Party members they are the best candidate to lead the UK out of the European Union, and possibly into an early election.
So what happens next?
Tuesday, June 25
Sky News had cancelled its planned head-to-head debate after Mr Johnson declined to attend.
But the broadcaster has said it is ready to host a debate tomorrow evening "if both candidates make themselves available".
If Mr Johnson does not change his mind, the debate will be delayed until July 1.
However there is no sign Mr Johnson would agree to the delayed debate.
Mr Hunt has called his rival a “coward” for avoiding public appearances during the leadership campaigns.
Wednesday, June 26 – Wednesday, July 17
The Conservative Party has arranged a series of 16 hustings across the country, including a 'digital hustings' on June 26.
The majority of these meetings with potential voters are being held in England, as well as one hustings each in Scotland, Northern Ireland and Wales.
Saturday, July 6
Conservative Party members should start receiving their postal ballots by this date. Each of the party’s 160,000 members gets a vote.
In order to prevent vote rigging, members must have joined the party before the leadership election process began.
Tuesday, July 9
ITV has announced a debate on this date featuring both candidates.
The BBC is also expected to hold a debate but is yet to announce a time and whether both Mr Hunt and Mr Johnson have agreed to attend.
Sunday, July 21
Party members must have returned their postal ballots by this date.
Tuesday, July 23
The 1922 Committee of Tory backbenchers will tally up the votes and announce a winner.
Theresa May could visit the Queen to resign immediately, or declare a brief transition period before her successor enters 10 Downing Street.
The victor will be invited to Buckingham Palace to form a government.
He then has 101 days to confirm what type of Brexit the UK will take.