The voting process takes about two weeks.
Voting papers are sent out to 380 Labour MPs and Members of the European Parliament, as well as 200,000 party members and 3.2m members of trade unions connected to the Labour Party.
These people help to make up what's called the electoral college - which is divided into three sections.
- MPs and MEPs
- Party members
- Trade union members
Even though some of the sections are made up of lots more people, the three groups each have one-third of the say in what happens.