In this chapter I introduce the central themes of this book. I reflect upon the prominent role of prison building programmes during the 2024 UK General Election and begin to situate these political debates within a broader context associated with prison expansionism in England and Wales since the mid-1990s punitive turn. I build the case for the systematic analysis of prison building programmes and seek to explain these penal developments with reference to the mediating role of the penal state. My central claim here is that prison building programmes are shaped by the structure of the state, its powers and capacities, but must also be understood as constitutive of broader state building projects. These capital capacities create possibilities for action in the penal field and generate powerful policy feedback effects that incentivise some policy options while discounting others. I conclude this chapter with an overview of the research aims and methodological choices that guided this study and present an overall plan for this book.