Candidates for leader and deputy leader have to be MPs, and they need nominations from 10% of Labour MPs and MEPs.
And in a new rule, candidates also need nominations from 5% of Labour's constituency parties.
Alternatively, they need nominations from three affiliated bodies, two of which must be trades unions, adding up to 5% of affiliated members.
Who can vote?
Members of the Labour Party, affiliated trades unions (if they opt in), and socialist societies such as the Fabians, all get one vote each.
In 2015, non-members were allowed to register as supporters and vote in the contest for a £3 fee.
Those new registered supporters voted overwhelmingly for Jeremy Corbyn, though he gained enough support from members and affiliates to win anyway.
In 2016, when Owen Smith challenged Jeremy Corbyn, the cost of registering was raised to £25 and people were given only two days to sign up.
The cost and the time period for registering this time will be in the hands of the NEC.