Tenth Amendment

• The Tenth Amendment of the U.S. Constitution states that all powers not explicitly granted to the federal government, or explicitly prohibited to the states, by the Constitution will be reserved to the states. The Tenth Amendment resulted from considerable debate between the Federalists and Anti-Federalists over the advisability of having a strong central government in the new United States. The two factions compromised by enumerating the powers of the federal government and theoretically leaving everything else to state governments.
  
  

Article IV

• Article IV of the U.S. Constitution contains a passage known as the Supremacy Clause. This clause makes treaties and federal law (including the Constitution) superior to any law made by the states. Therefore, if any state law should operate in conflict with these other types of law, courts will not enforce the state law.
  
  

Express Preemption

• The process by which a federal law trumps a conflicting state law is known as preemption. Express preemption is the simplest case, in which language in the federal statute acts to invalidate the state statute. For instance, if a federal statute specifically says that states may not forbid their citizens from wearing yellow pants, any state statute currently in existence that contains such a prohibition will be expressly preempted.
  
  

Implied Preemption

• Implied preemption is not so clear-cut as express preemption. Implied preemption occurs when existence of a disparate state statute would seem to undermine the objectives of a federal statute. There are two types of implied preemption. The first, conflict preemption, occurs when a state statute is so different from a federal statute that one individual could not comply with both statutes at once. The second type of implied preemption is field preemption, where the federal statute expresses a clear intention to dominate the entire field such that there is no room for state law. For instance, if a federal statute says "Any citizen may wear any color pants he likes" then any state law dealing with the color of pants would theoretically be preempted.
  
  

Restrictions on States

• The Constitution makes certain areas of law inappropriate for state statutes. For instance, state governments may not make any law that harms interstate commerce, nor may a state enter into an agreement with a foreign government. The Constitution also establishes requirements for certain types of state law to receive federal government consent, such as a state law to tax federal government property, agreements between state governments or taxation on imports.