In this chapter I explore the persistence of high incarceration rates in England and Wales since the mid-1990s punitive turn. Building upon recent theoretical work on path dependency, I identify prison population and estate capacity forecasting as a poorly understood policy feedback mechanism that helps to determine the overall scale, scope and reach of the prison state by connecting the politics of prison building programmes with ‘business as usual’ planning cycles that assume considerable policy continuity with the past. I illustrate this point with reference to recent controversies over women’s imprisonment where the everyday, routinised working practices of penal elites have played an important role in sustaining prison expansionism long after the initial conditions that fuelled the prison boom have faded. Disrupting these self-fulfilling logics will not be easy and I conclude this chapter with a call for a more deliberative democratic politics that confronts penal momentum and invites greater strategic reflection on the past, present and possible futures of our penal system.