This paper investigates corporate income tax planning at the state level. Specifically, we examine whether certain firm-specific characteristics ? the number of states in which corporations file tax returns and their state apportionment factors ? impact firms' overall state tax burdens. The analysis is based on confidential survey data from firms included in the IRS's Coordinated Examination Program that are rich in certain state-related data not available publicly for individual firms. We find that firms' state tax burdens decrease then increase in the number of states in which they file returns and are minimized at an estimated 21 states. Using the best available data to explore firm-level apportionment opportunities, we find our proxy for sales factor apportionment is related to lower state tax burdens, although this result is sensitive to sample composition. With regard to investments in state tax planning, we find that the average payoff of 100 to 1 is much higher than has been previously documented for federal tax planning.