Studies of a small number of countries have revealed that both democratic and nondemocratic subnational governments can exist within a single country. However, these works have neither demonstrated how common subnational regime variation is nor explained why some countries are more prone to it. This article does both. We show that subnational regime variation exists in all world regions, in both unitary and federal states, and in both the present and past, using Varieties of Democracy global data from 1900 to 2018. The article also demonstrates theoretically and empirically how social heterogeneity and factors undermining a national government's ability to extend control throughout a country promote this variation. Specifically, subnational regime variation is more common in countries that are ethnically diverse, rugged, and populous.