Over the several decades, observers of American politics have noted the sharp increase in partisanship and ideological polarization among members of Congress. While better ideological differentiation may provide voters clearer choices and increase accountability, the results of recent partisan and ideological battles have raised questions about the impact pf polarization on good governance. While much scholarly effort has gone into studying the root causes on congressional polarization, such research has been hampered by its sole reliance on the US House and Senate for data on legislative polarization. But new data on polarization of state legislatures provided by Shor and McCarty (2011) and updated with the generous support of the John and Laura Arnold Foundation expands our capacity to uncover the political, economic, and social factors that underlie our increasingly polarized system. In this report, we review the evidence concerning the polarization of the US Congress and supplement it with analyses based on the experience of polarization in the US states. We show that while there is variation in polarization across states, in aggregate the patterns are very similar to the national experience. Moreover, analyses of the causes of polarization at the national level are generally confirmed by the data on the states. The richer data from the sates, however, allows us to address new sets of questions which suggest some limited opportunities for reforms targeted at reducing polarization.