What legal principle determines if laws or actions by the government are constitutional?

The legal principle that determines if laws or actions by the government are constitutional is known as judicial review. This doctrine allows courts, particularly the Supreme Court, to evaluate the validity of legislative acts and executive actions against the Constitution. When a law is challenged, courts will assess whether it aligns with constitutional provisions, and if it does not, the law can be declared unconstitutional, which effectively nullifies it.

Judicial review plays a crucial role in maintaining the checks and balances within the government by ensuring that no branch exceeds its authority or enacts laws that violate constitutional rights. This principle was established in the landmark case Marbury v. Madison in 1803, highlighting its foundational importance in American constitutional law.

In contrast, statutory law refers to the written laws enacted by the legislature, while common law is derived from judicial decisions and precedents. Judicial activism describes a judicial philosophy where judges are viewed as being more willing to make policy decisions or interpret the law in a progressive manner, rather than strictly applying the law as written. None of these directly address the mechanism by which laws are evaluated for constitutionality, which is the essence of judicial review.