The multiple streams model, developed by Kingdon in the United States (US), is being adapted increasingly to study European Union (EU) policy making. This, however, is revealing a theoretical underdevelopment in some of its central components. The present paper considers several concerns. It seeks to develop the idea of policy entrepreneurship as a context-specific activity that gives substance to the claim that 'ideas have their time'; it interprets the key notion of ambiguity, in the EU context, to mean institutional ambiguity; and it allows for spillovers between policy areas to be endogenous as well as exogenous. This affects the nature of the policy windows wherein policy entrepreneurship occurs. The adapted multiple streams model is used to analyse the 2005 EU sugar policy reform. Institutional ambiguity and endogenous spillovers are shown to create the conditions that enabled active policy entrepreneurship to lead, ultimately, to reform of this most resilient of policies.