The Constitution of the United States is made up of hundreds of clauses. Article VI is known as the Supremacy Clause and it states that federal law is “the supreme law of the land.” This means that state laws cannot contradict or interfere with federal laws. While the Constitution’s Supremacy Clause states that federal law reigns supreme, dual sovereignty exists in the United States. Dual sovereignty means that states have the power to enact their own laws, and state and federal laws can coexist. Congress can also include savings clauses when passing a law, which preserves the state’s ability to create laws and allow states to fill regulatory voids or enhance certain protections as necessary. For example, the Occupational Safety and Health Administration (“OSHA”), which administers the Occupational Safety and Health Act, allows states to create their own workplace health and safety laws as long as they are “at least as effective as OSHA in protecting workers and in preventing work-related injuries, illnesses, and deaths.”

In instances where state laws conflict or interfere, federal laws preempt state law. When preemption occurs, the state law is invalidated by federal law, and the state law can no longer be enforced. It is the judicial system’s job to determine whether a law is preempted, and it does so on a case-by-case basis. The Supreme Court of the United States has identified two main types of preemption: express and implied. Both are discussed in more detail below.

Express Preemption

Express preemption occurs when a federal statute or regulation contains language that explicitly says that the law preempts state law. Several federal laws exist that contain express preemption clauses.  While the concept of express preemption is relatively simple, there has been debate over what the federal laws expressly preempt. In some cases, the law expressly preempts any form of state laws that affect the federal law, in other cases certain types of state laws might be allowed. The language of the federal laws is not always clear and it is up to the courts to interpret it. In National Meat Association v. Harris, the United States Supreme Court addressed this very issue. The National Meat Association argued that the Federal Meat Inspection Act (“FMIA”) prevented California from imposing its own state requirements on federally inspected slaughterhouses. The Supreme Court held that the FMIA contains an express preemption clause which applies to laws that a state might impose on slaughterhouses that were in addition to, or different than what the FMIA requires. The FMIA preemption language that the Supreme Court referred to states that “[r]equirements within the scope of [the FMIA] with respect to premises, facilities and operations of any establishment at which inspection is provided under . . . [the FMIA], which are in addition to, or different than those made under [the FMIA] may not be imposed by any State. The California law was preempted by the federal FMIA because it imposed requirements on federally inspected slaughterhouse that were different than those required by the FMIA.

The FMIA is not the only federal law that contains express preemption clauses. For example, the Nutritional Labeling and Education Act, which amended the Food, Drug, and Cosmetic Act (“FDCA”), expressly preempts any state law requirement for food labeling that is “not identical” to a requirement of the FDCA. Another example of express preemption can be found in the Poultry Products Inspection Act (“PPIA”). The PPIA contains a preemption provision much like the FMIA that does not allow states to create laws different from the PPIA. The Federal Insecticide, Fungicide, and Rodenticide Act also contains provisions that expressly preempt states from creating their own pesticide labeling and packaging laws.

Implied Preemption

Implied preemption occurs if it can be shown that Congress intended to preempt state law when the federal law was created. There are two different types of implied preemption, field preemption and conflict preemption. Regardless of the type of implied preemption at issue, determining Congress’s intent can be difficult, especially when federal laws give states some amount of authority.

Field Preemption

Even if a federal law does not expressly preempt state law, a court may find that Congress intended the federal law to occupy the entire “field” of an issue. This is known as field preemption. When a law is found to “occupy an entire field,” that means that Congress has passed comprehensive, all-encompassing legislation that effectively covers everything within the federal law’s specific regulatory power. Field preemption leaves no room for additional state laws, impliedly preempting them by the adoption of the broad federal law. Courts tend to find field preemption most often when federal law is the more appropriate regulatory approach, largely in issues that involve the crossing of state lines. This includes laws dealing with labor, immigration, environment, and agriculture.

In 1947, the Supreme Court found that federal laws regarding grain warehousing preempted state regulations in Rice v. Santa Fe Elevator Corporation. The Supreme Court found that while there was no express preemption, Congress had intended to occupy the entire field of grain warehouse regulation when it passed the Warehouse Act. The Supreme Court was able to find that Congress intended for the federal Warehouse Act would preempt state grain warehouse laws. This was largely because of the text and legislative history associated with the Warehouse Act, in which the House Committee stated the purpose of the Warehouse Act was to make it “independent of any State legislation on the subject.”

Conflict Preemption

The other form of implied preemption is conflict preemption. Conflict preemption occurs when a conflict arises between a state and federal law.  In general, conflicts between state and federal law are most likely to arise when a federal law provides authority for states to enact their own laws that work in conjunction with the federal law. Conflict preemption is broken down into two categories, impossibility preemption and obstacle preemption.

Impossibility preemption occurs when it would be impossible for someone to comply with both federal and state law. The Supreme Court addressed this type of preemption in Florida Avocado Growers v. Paul. In this case, California had prohibited the sale of avocados within the state if they did not meet the state standard of maturity, which was based on oil content. However, a federal standard for determining the maturity of avocados also existed, which did not consider oil content. Producers of avocados in Florida argued that the conflict between the state and federal avocado maturity standards was preempted. The Supreme Court found that the federal law did not preempt the state law because producers could comply with both standards. The Supreme Court also found that if it becomes physically impossible to comply with both federal and state laws, the federal law would preempt the state law.

Obstacle preemption occurs when a state law becomes an obstacle to federal law. A state law becomes an obstacle when it interferes with Congress’s purpose for the law. For example, in In Re: Aurora Dairy Corp, the Eighth Circuit examined whether the Organic Food Production Act (“OFPA”) preempted the laws of several states in a class action lawsuit. The plaintiffs in the lawsuit wanted to prevent Aurora Dairy from labeling their products as “organic” because they claimed it was misleading under the laws of several states. The court found that, because Aurora did obtain certification, the state laws at issue did pose as an “obstacle to the accomplishment of congressional objectives.” Because if the court had let the lawsuit proceed, the state law would have undermined Congress’s purpose for the OFPA of creating a national standard for defining organic food, the state was subject to obstacle preemption.


Overall, Congress has the power to control how much authority states have when it comes to enacting new laws. In most cases those laws can exist harmoniously. However, as discussed above, when conflict occurs, federal law preempts it all.


For more information on preemption under FIFRA, click here.

To learn more about the regulatory process, click here.