Implies in the commerce clause that, because Congress has power to regulate interstate commerce, individual states are limited in their ability to legislate on such matters.
If Congress has not enacted legislation in a particular area of interstate commerce, the states are free to regulate, so long as the state statutes:
- do not discriminate against out-of-state commerce
- do not unduly burden interstate commerce
- do not regulate extraterritorial (wholly out-of-state) activity.
Does not apply where state acts as a market participant.