To understand why successive governments have chosen to invest in new prison building we must first interrogate the political pre-decision processes of agenda setting and alternative specification. Drawing upon John Kingdon’s pioneering work on the multiple streams framework, I begin to pry open the ‘black box’ of contemporary penal policymaking and explore the forces that have propelled prison building programmes on to the political agenda in England and Wales since the mid-1990s punitive turn. I present findings from my thematic analysis of qualitative interviews with penal policymakers and describe a prison building ‘garbage can’ that is comprised of four main elements: a drip feed of problem indicators; a new politics of supply and demand, the constructive ambiguity of prison building and departmental policy entrepreneurship. I conclude this chapter by reflecting on the importance of agenda setting activity and how the coupling of competing ideas, rationalities and justifications for prison building sets the tone for policy implementation.