The article by Simmons, Dobbin, and Garrett explores the international diffusion of economic and political liberalism in the late 20th century. It documents the widespread adoption of free-market reforms and democratization policies globally and proposes four theories to explain this diffusion: coercion, competition, learning, and emulation. The authors argue that national policy choices are interdependent, influenced by the actions of other countries and international actors. They review the patterns of liberalization, distinguish between interdependent and independent decision-making, and elaborate on the diffusion hypotheses. The article aims to bring together different schools of thought and define common metrics to judge the effectiveness of these approaches in different policy domains. The symposium articles that follow test these theories empirically, examining how countries' policy choices are affected by the choices of others, sometimes mediated by international organizations and private actors. The authors highlight the importance of understanding the mechanisms of diffusion to explain the timing and geographic reach of liberal innovations.The article by Simmons, Dobbin, and Garrett explores the international diffusion of economic and political liberalism in the late 20th century. It documents the widespread adoption of free-market reforms and democratization policies globally and proposes four theories to explain this diffusion: coercion, competition, learning, and emulation. The authors argue that national policy choices are interdependent, influenced by the actions of other countries and international actors. They review the patterns of liberalization, distinguish between interdependent and independent decision-making, and elaborate on the diffusion hypotheses. The article aims to bring together different schools of thought and define common metrics to judge the effectiveness of these approaches in different policy domains. The symposium articles that follow test these theories empirically, examining how countries' policy choices are affected by the choices of others, sometimes mediated by international organizations and private actors. The authors highlight the importance of understanding the mechanisms of diffusion to explain the timing and geographic reach of liberal innovations.