From the 1950s to the 1990s, the “incrementalist” model dominated theories of policymaking. Since Baumgartner and Jones's Agendas and Instability in American Politics was published in 1993, however, the “punctuated equilibrium” model, borrowed from evolutionary biology, has supplanted the incrementalist model. But what do such alien models do to the discipline of political science, and what does the discipline do to the models, when they are thus imported? In this article, I first discuss the functions of concepts in political science, then discuss and analyze the creation and meaning of the concept of punctuated equilibrium in biology, then trace its transformation as it was initially adopted by political scientists, then analyze the manner in which its meaning developed and altered as it grew in popularity within the discipline, and finally assess its value to political science now. I will conclude with some general observations about the process of importing concepts from outside the discipline.