Beginning with the great case of Union Transit Co. v. Kentucky, the Supreme Court has steadily advanced the principle that a state lacks power to tax property located out of the state. But the law, as to taxation of acts done out of the state, has not been so well worked out. It is believed that a recent decision of the court is of importance in this field. But first, a very brief review will be made of the more important cases marking steps in the working out of the problem.