Constitutional law is a body of law that defines the role, powers, and structure of different entities within a state, including the executive, the parliament or legislature, and the judiciary. It deals with the fundamental principles by which the government exercises its authority. Constitutional law is a major focus of legal studies and research.

Constitutional law usually refers to rights granted by the U.S. Constitution. Cases often involve the Bill of Rights, which contains the first 10 amendments to the U.S. Constitution. These amendments contain such rights as the freedom of speech, the right to a fair trial, and the right to be free from certain types of discrimination. States also have their own constitutions, which usually contain most, if not all, of the same rights guaranteed under the U.S. Constitution.

Constitutional law also involves the rights and powers of the branches of government. Both the federal and state constitutions outline three branches of government and give distinct powers and responsibilities to each one. Constitutional lawyers also help resolve disputes among the branches.

The Constitution is the source of legal authority for the United States, and questions of constitutional law often relate to fundamental questions of sovereignty and democracy. The Supreme Court has the authority to conclusively decide questions of constitutional law through their power of judicial review. Judicial review allows courts to declare actions of governmental bodies unconstitutional and, therefore, prevent the actions from being enforced. Consequently, the study of constitutional law focuses heavily on Supreme Court rulings.