Preemption is a doctrine in law which states that if there is a conflict between laws in a lower level of government and laws in a higher level of government, the higher laws win. In a simple example of preemption, if a law in the State of Illinois conflicts with Federal law in the United States, the Federal law is considered the law which must be obeyed. Preemption can come in a number of different forms, such as conflicts between municipal and state or provincial law, conflicts between laws in European Union member states and the Law of the European Union, and so forth.
Different nations have different rules about preemption. As a general rule, there are two basic types: Explicit and implied. Explicit preemption involves a law on a higher level which explicitly states that it is intended to preempt laws on a lower level, as for example when the federal government in the United States passed civil rights laws which explicitly overturned discriminatory laws in the states. In implied preemption, a conflict exists, but the higher law doesn't spell out the stipulation that it is supposed to supplant the lower law. Cases of implied preemption can sometimes be open to interpretation.
Within laws which are characterized by implied preemption, there are conflict versus field preemptions. Conflict covers a situation in which two laws contradict each other, or in which one law interferes with the enforcement of another. For example, if a law in Germany interferes with the enforcement of a law at a higher level in the European Union, it can be challenged under the preemption doctrine.
Field preemption has to do with situations in which higher order laws are intended to the primary form of governance in a particular field to avoid conflict and confusion. Immigration law, for example, is usually written on a federal level and the federal government does not want states or provinces to pass their own versions of immigration law. These types of situations often arise with laws which affect national security.
This doctrine can make things challenging for people navigating the legal system. They may find that after they take cases to court on a state or provincial level, the national government may throw the case out if they attempt to appeal a verdict because it does not recognize the state regulations which formed the foundation of the case. Likewise, national laws may make it difficult to take certain types of cases to court, such as cases against national agencies, even if state laws allow citizens to bring such suits.