Over the course of the twentieth century, a broad array of parties as organizations of a new type took over state functions and replaced state institutions on the territories of the former Ottoman, Qing, Russian, and Habsburg Empires. In the context of roughly simultaneous imperial and postimperial transformations, organizations such as the Committee for Union and Progress (CUP) in the Ottoman Empire (one-party regime since 1913), the Anfu Club in China (parliamentary majority since 1918), and the Bolshevik Party in Russia (in control of parts of the former empire since 1918), not only took over government power but merged with government itself. Disillusioned with the outcomes of previous constitutional and parliamentary reforms, these parties justified their takeovers with slogans and programs of controlled or supervised economic and social development. Inheriting the previous imperial diversities, they furthermore took over the role of mediators between the various social and ethnic groups inhabiting the respective territories. In this respect, the parties appropriated some of the functions which dynastic and then constitutional and parliamentary regimes had ostensibly failed to perform. In a significant counter-example, in spite of prominent aspirations, no one-party regime emerged in Japan, for there the constitutional monarchy had survived the empire’s transformation to a major industrialized imperialist power. One-party regimes thrived on both sides of the Cold War and in some of the non-aligned states. Whereas several state socialist one-party regimes collapsed in 1989–1991, some of the communist parties have continued to rule, and new parties managed to monopolize political power in different Eurasian contexts.