There are two ways a Conservative Party leadership contest can be triggered.
The first is if the current leader resigns.
The second is if 15% of Conservative MPs write to the chair of the backbench 1922 Committee saying they no longer have confidence in the leader.
A vote of no confidence is then held, with Conservative MPs voting in support or against the leader.
A leader who loses a confidence vote is not allowed to take part in the subsequent leadership contest.
The contest takes place in two stages.
In the first stage, Conservative MPs put themselves forward as candidates.
All Conservative MPs then vote in a series of rounds to reduce the number of candidates until only two remain.
The second stage of the contest sees the two remaining candidates put to a vote of Conservative Party members.