There is no single body of law in the United States. Instead, fifty state legal systems sit beneath a federal system, and local ordinances sit beneath both. Determining which level controls a question is frequently the first practical move in legal research, since identical conduct may be regulated differently depending on where it occurs.
The Constitutional Design
The federal government holds only limited, enumerated powers. Article I, Section 8 of the U.S. Constitution lists Congress's authority, covering interstate commerce, bankruptcy, patents and copyrights, naturalization, and federal taxation, among other items. The Tenth Amendment keeps powers not given to the federal government with the states or the people. States, for their part, possess a general police power to protect health, safety, and welfare, and that power underlies most criminal, family, and land-use law.
Situations Where Federal Law Governs
Federal law controls the areas the Constitution commits to the national government: immigration, bankruptcy, federal crimes, patents and copyrights, securities regulation, federal tax, and matters tied to interstate commerce. Federal agencies also adopt regulations that carry the force of law, gathered in the Code of Federal Regulations (ecfr.gov). Federal courts hear cases arising under the Constitution and federal statutes, and they also decide certain disputes between citizens of different states.
Situations Where State Law Governs
States lead on family law, marriage and divorce, wills and probate, property, contracts, most crimes, traffic, professional licensing, and education. Every state maintains its own constitution, statutes, and court rules. That explains why a contract clause, a custody rule, or a criminal sentence can vary from one state to another, and why lawyers so often answer that the law depends on the jurisdiction.
Overlap Between the Two Systems
Many everyday problems touch both systems at once. Employment discrimination, for example, may fall under federal civil rights statutes and under a broader state anti-discrimination law. Consumer protection, environmental regulation, and wage rules routinely have both federal and state layers. A person might bring claims under either or both, and the forum chosen can change procedure, remedies, and timing.
The Supremacy Clause and the Doctrine of Preemption
Article VI's Supremacy Clause establishes the Constitution, federal statutes, and treaties as the supreme law of the land, binding state judges even when state law says otherwise. Preemption is the doctrine describing when federal law displaces state law. It arises in several forms:
- Express preemption: the federal statute states outright that it overrides state law.
- Field preemption: federal regulation is so thorough that it leaves states no room to act.
- Conflict preemption: a state law makes compliance with both impossible, or blocks the federal objective.
Preemption is never automatic. Courts start from a presumption that states may exercise their traditional police powers, and they examine the statute's text and structure to determine whether Congress meant to displace state law.
Choice of Law and Full Faith and Credit
Article IV's Full Faith and Credit Clause obliges states to honor the public acts, records, and judicial proceedings of other states, which is why a judgment issued in one state can generally be enforced in another. When a dispute spans several states, courts apply choice-of-law rules to determine whose substantive law applies. Those rules differ from state to state, so the parties' locations, the events, and the contract terms can each matter.
Which Court Has Jurisdiction
Subject-matter jurisdiction decides whether a case belongs in federal or state court. Federal courts hold federal-question jurisdiction over claims arising under federal law and diversity jurisdiction over disputes between citizens of different states when the amount exceeds a statutory threshold, currently $75,000 under 28 U.S.C. § 1332. State courts are courts of general jurisdiction and hear most disputes. A defendant can sometimes remove a case from state court to federal court when federal jurisdiction exists.
| Feature | Federal law | State law |
|---|---|---|
| Basis of authority | U.S. Constitution and enumerated powers | State constitution and general police power |
| Common subjects | Immigration, bankruptcy, patents, federal tax, interstate commerce | Family, property, contracts, most crimes, traffic, licensing |
| Court structure | U.S. district courts, courts of appeals, Supreme Court | State trial and appellate courts |
| Research sources | uscode.house.gov and ecfr.gov | The state legislature's official code site |
State Constitutions May Go Further
A state constitution can protect rights more broadly than the U.S. Constitution does, and state courts may construe their own charters independently. Where a federal right sets a floor, a state can sometimes build above it. That is another reason to examine state law even when a federal question is in play.
Local Ordinances: A Third Layer
Below state law lies local law. Cities, counties, and special districts enact ordinances covering zoning, noise, building permits, business licensing, animal control, and similar day-to-day concerns. Local rules may not conflict with a valid state statute or the state constitution, and states can preempt local regulation in particular areas. Even so, an ordinance may impose requirements that state law does not, so a complete answer often means checking the municipal code alongside the state code.
Overlapping Federal and State Agencies
Overlap is routine at the agency level. Environmental permits, workplace safety, food and drug labeling, and consumer financial products can all involve federal regulators and their state counterparts. A federal agency sometimes delegates primary enforcement to a state program that must satisfy federal minimum standards. Knowing which agency holds authority, and which remedies it can impose, can matter as much as knowing which legislature wrote the underlying statute.
Researching at the Correct Level
- Name the subject, then ask whether it is traditionally federal, state, or shared.
- For federal law, search the U.S. Code at uscode.house.gov and regulations at ecfr.gov.
- For state law, use the state legislature's official code site.
- For local rules, consult the city or county code.
- Where layers overlap, compare the remedies and deadlines each one offers.
Untangling federal, state, and local authority confuses many researchers, and the answer can change the outcome. If a real dispute or deadline is at stake, confirm the governing law with a licensed attorney. This guide is informational only and is not legal advice.