Part I of this article examines the structure and underlying policy of the Uniform Division of income for Tax Purposes Act's provisions relating to the apportionment of income arising from economic activity conducted across state lines. In particular, it considers the Act's "throwback" rule, which reapportions income ordinarily apportioned to a state in which it is not taxable to one in which it is. Part II explores in detail the Illinois court's resolution of the problem raised by GTE, namely, how to assign sales of tangible personal property, which are used as a basis for apportioning income, when such sales are effected by a taxpayer whose income is taxable in neither the state of the sales' origin nor that of their destination. Part III suggests an alternative resolution of the controversy that serves the relevant statutory and constitutional interests more fully than the Illinois court's opinion, while ensuring that all of a taxpayer's income is taxable by the states.