Quality-Adjusted Life-Years (QALYs) are central to healthcare decision-making in Britain and abroad, yet their history is poorly understood. In this paper, we argue that a more in-depth and political history of the QALY is needed to allow a critical evaluation of its current dominance. Exploiting rich data from archives and 44 semi-structured interviews conducted between 2015 and 2018, we employ Multiple Streams Analysis to construct a complex and dynamic picture of how the idea of QALYs emerged and was adopted within UK health policy. Through its historical and political approach, the paper illuminates the relative roles in the policy-making process of experts (especially economists) and politicians as 'entrepreneurs' in the development of new ideas; how these were influenced by negotiation within established and emerging institutional structures; and the role of serendipity and crisis.