Why have Eastern European states adopted flat tax policies? That is what this dissertation answers. This is a curious development given that flat tax policies were noticeably absent from the landscape of most of the world, including Eastern Europe. Fives cases of adoption are examined, including Estonia, Latvia, Lithuania, Slovakia and the Czech Republic. I argue that two simultaneous processes occur in Eastern Europe that makes adoption viable. First, at the domestic level, the idea of the flat tax is held in esteem by a number of actors, specifically: elite carriers, tax and financial ministers, think tanks and right-wing political parties. They champion this idea to its adoption, or at the least, introduce the flat tax into the policy-making apparatus. Second, at the international level, policy diffusion of the flat tax is taking place. In other words, the experience of previous adopters impacts the decisions of future adopters. Examining both cognitive heuristics theory and rational learning I argue that there are “varieties of diffusion” during the diffusion of the flat tax. Additionally, though this dissertation concerns itself primarily with adoption, I also investigate two cases of non-adoption in Poland and Hungary. What is argued is “diffusion without adoption” occurs. The idea of the flat tax diffused, but the adoption was not politically, ideologically, and economically feasible.