U.S. state legislatures vary substantially in the degree to which they are professional institutions. Drawing on the state policy literature, this study identifies three general categories of potential influence on this variation: the characteristics of a state's population, its governmental structures, and the level of legislative professionalism in its peer group states. Using four measures of professionalism from different periods, the author tests hypotheses about several such variables in cross-sectional and panel analyses. He finds that each of these categories of influence has significant, independent effects on legislative professionalism, and that taken together these influences account for most of the observed variation in professionalism across the states.