What happens when federal and state law conflict?

The Supremacy Clause in Article VI makes the Constitution and valid federal law the supreme law of the land, so where a state law genuinely conflicts with a constitutional federal law, the federal one controls and the state provision is preempted. Preemption can be express, where Congress states it directly, or implied, where federal regulation is so comprehensive that it occupies the field or where compliance with both is impossible. Courts do a great deal of work determining which category applies, and the answers are frequently contested.

The counterweight is that federal power is enumerated. The Tenth Amendment reserves to the states everything not delegated to the federal government, and states hold what is called the police power, general authority over health, safety, welfare, and morals. That is why most criminal law, family law, contract and property law, professional licensing, and education policy are state matters, and why moving across a state line changes the rules on marriage, inheritance, gun ownership, and speed limits. States can also be more protective than federal floors, offering stronger consumer, labor, or environmental standards than federal minimums require. The genuinely awkward zone is where federal and state policy diverge without a court resolving it, most visibly with marijuana, legal under many state laws and still a controlled substance federally, which creates real practical problems for banking, employment, and interstate travel.