What types of firms establish tax haven operations, and what purposes do these operations serve? Analysis of affiliate-level data for American firms indicates that larger, more international firms, and those with extensive intrafirm trade and high R and D intensities, are the most likely to use tax havens. Tax haven operations facilitate tax avoidance both by permitting firms to allocate taxable income away from high-tax jurisdictions and by reducing the burden of home country taxation of foreign income. The evidence suggests that the primary use of affiliates in larger tax haven countries is to reallocate taxable income, whereas the primary use of affiliates in smaller tax haven countries is to facilitate deferral of U.S. taxation of foreign income. Firms with sizeable foreign operations benefit the most from using tax havens, an effect that can be evaluated by using foreign economic growth rates as instruments for firm-level growth of foreign investment outside of tax havens. One percent greater sales and investment growth in nearby non-haven countries is associated with a 1.5 to 2% greater likelihood of establishing a tax haven operation.