Substantive due process is a constitutional doctrine that protects certain fundamental rights from government interference. It is derived from the Fifth Amendment and the Fourteenth Amendment of the U.S. Constitution, which guarantee that no person shall be deprived of life, liberty, or property without due process of law. This principle ensures that laws and government actions are fair and reasonable, particularly concerning individual rights. In practice, substantive due process often involves judicial review of laws that may infringe on personal freedoms, such as the right to privacy or the right to marry. Courts assess whether the government's actions serve a legitimate purpose and whether they are necessary to achieve that purpose, balancing individual rights against state interests.