The G20 is not readily explained by conventional theories of international organizations. I suggest that the approach of historical institutionalism provides a set of insights that help to better understand the G20. This chapter first introduces five innovative analytical insights that historical institutionalism (HI) brings to the study of institutions, and then it applies HI to explain three core puzzles: (1) the emergence and design choice of the G20, (2) its particular technique of exercising governance authority, and (3) the institutional and regulatory trajectory taken by the G20. An HI analysis cautions skepticism towards the claim that the G20 is transformative, emphasizing instead its historical legacy, institutional embeddedness, and self-reinforcing dynamics that do not favor a radical shift away from the status quo. Instead, the institutional dynamics of the G20 inhibit a move to deeper and more radical policy reform.