This document provides an overview of three models of how policy innovations diffuse between states in the US: competitive, idiosyncratic, and collegial. The competitive model views states as adopting innovations to gain competitive advantages over other states. The idiosyncratic model stresses that innovations are often tailored to a state's unique circumstances and do not easily transfer elsewhere. The collegial model sees states learning from each other in a cooperative manner. The document discusses examples of each model and examines how the rate and pattern of innovation diffusion affects the ability of federalism to promote policy experimentation across states.