Despite the large and complex conformational space available to an unfolded protein, many small globular proteins fold with simple two-state cooperative kinetics. Understanding what determines folding rates beyond simple rules summarizing kinetic trends has proved to be more elusive than predicting folding mechanism. Topology-based models with smooth energy landscapes give reasonable predictions of the structure of the transition state ensemble, but do not have the kinetic or thermodynamic cooperativity exhibited by two-state proteins. This review outlines some recent efforts to understand what determines the cooperativity and the diversity of folding rates of two-state folding proteins.