Which crimes are considered federal and which are considered state? There is no clear answer to this question. Indeed, criminal conduct cannot be sorted into these two baskets. When a single act or course of conduct violates both federal and state criminal laws, it is even possible for both governments to prosecute because, under the "dual sovereignty" doctrine, the double jeopardy prohibition (according to which a person may not be tried twice for the same offense), does not apply to separate prosecutions by separate sovereigns.

In theory, congressional power is limited to the powers expressly enumerated in Section 1 of the Constitution. Offenses like counterfeiting U.S. currency, illegally entering the United States, treason, and violation of constitutional and federal statutory rights are obviously within the federal government's core jurisdiction. But, utilizing its expansive powers under the commerce clause and other elastic provisions, Congress has passed federal criminal laws dealing with drug trafficking, firearms, kidnapping, racketeering, auto theft, fraud, and so forth.

The Supreme Court has rarely found that Congress lacked authority to pass a federal criminal law. Partially because of this, the reach of federal criminal law grew inexorably throughout the 20th century. Today, federal criminal law can be used to prosecute many offenses that traditionally were regarded as a state responsibility. In practice, however, the great constraint on the reach of federal criminal law is resources. The FBI and other federal law enforcement agencies, as well as federal prosecutors, can investigate and prosecute only a small fraction of all the crimes that potentially fall within their purview.