This paper examines alternative explanations for the decline over the past two decades in state corporate income taxes relative to the state economy. We employ a survey of state tax administrators, individual tax returns from Georgia and Utah, and panel data to explore the importance of tax policy, tax planning, and economic factors on the trend in state corporate taxes. We find that corporate tax planning and economic factors account for much of the relative decline and that state tax policy changes are important factors. However, federal tax changes had only a modest effect during this period.