Every state that levies a corporate income tax must determine, for each company doing business within its borders, how much of the company’s profits it can tax. One factor that all such states use to make this determination is the percentage of the company’s nationwide sales that can be attributed to the state. Ideally, all of a company’s sales would be attributed to the states in which it operates, but, due to differences among states’ corporate income tax rules, this is not always the case. In some instances, a portion of a business’ sales are not attributed to any state, which means that a corresponding portion of its profits go untaxed, a phenomenon often referred to as “nowhere income.” This policy brief explains how this phenomenon arises and discusses how a throwback rule can be used to ensure that all corporate profits are subject to taxation.