In this chapter, I examine the significance of the sociological institutionalist (SI) ontology and epistemology underlying Patsy Healey’s ground-breaking work since the 1990s, and highlight the distinctive and valuable insights this approach yields through a comparison with a historical institutionalist (HI) perspective. The premise is that, as Healey has shown so clearly, ideas are powerful agents of change, and planning theory is not merely a meta-conversation separate from everyday life, but is a part of planning culture that shapes the way we understand both routine and complex processes, and has powerful impacts on the kinds of questions we ask, and how we produce answers that help us to think and to act.