Momentum is a British left-wing political organisation which has been described as a grassroots movement supportive of the Labour Party; since January 2017, all Momentum members must be members of the party. It was founded in 2015 by Jon Lansman, Adam Klug, Emma Rees and James Schneider after Jeremy Corbyn's successful campaign to become Labour Party leader and it was reported to have between 20,000 and 30,000 members in 2021.

Jonathan Lansman is a British political activist. He is best known for having worked on Jeremy Corbyn's successful 2015 campaign for the leadership of the Labour Party and subsequently founded the pro-Corbyn organisation Momentum. He is a member of the Labour Party's National Executive Committee. Lansman has worked for both Tony Benn and Michael Meacher, and was a prominent supporter of Benn in the early 1980s.