Though the U.S. federal investment tax credit (ITC) was permanently repealed in 1986, state-level ITCs have proliferated over the last few decades. The proliferation of state ITCs and other investment tax incentives raises two important questions: (1) Are these tax incentives effective in achieving their stated objective, to increase investment within the state?; and (2) To the extent these incentives raise investment within the state, how much of this increase is due to investment drawn away from other states? To begin to answer these questions, we construct a detailed panel data set for 50 states for 20+ years (depending on the series). The data set contains series on output and capital, their relative prices, and the number of establishments. The effects of tax parameters on capital formation and establishments are measured by the Jorgensonian user cost of capital that depends in a nonlinear manner on federal and state tax parameters. Cross-jurisdiction differences in state investment tax credits and state corporate tax rates entering the user cost, combined with a panel that is long in the time dimension, are key to identifying the effectiveness of state investment incentives. Three models are estimated: (1) a Capital Demand Model motivated by the first-order condition for profit-maximization; (2) a Spatial Discontinuity Model developed by Holmes (1998) that exploits the spatial discontinuity in tax policies that occurs at state borders; and (3) a Twin-Counties Model that matches counties to a cross-border "twin" and relates between county differentials in manufacturing activity to between-county differentials in tax policy. The first model relies on state-level data, while the latter two use county-level data. On balance, the models find a significant channel for state tax incentives on own-state economic activity and document the importance of interstate capital flows, a necessary element for meaningful tax competition. Whether state investment incentives are a zero-sum game among the states is less certain and depends on the definition of the set of competitive states.