This paper examines the British Labour Party's leadership election of 2015, which resulted in the unexpected victory of the radical-left candidate, Jeremy Corbyn. It looks at the contest using Stark's (1996) academic model of leadership elections, based on the tripod of selection criteria, acceptability, electability and competence, and finds it wanting. Selection rules, which are downplayed in Stark's model, are then examined, as Labour used a new selection system based on one-member-one-vote in 2015. While these are found to have had some impact, Corbyn's victory cannot be explained primarily by institutions. The paper reconsiders Stark's model and shows that it failed because of the diminished significance of electability as a selection criterion in the Labour leadership contest of 2015. That largely reflected the circumstances in which the contest took place, in the aftermath of a demoralising election defeat for Labour.