Constitutional law is a body of law which defines the role, powers, and structure of different entities within a state, namely, the executive, the parliament or legislature, and the judiciary; as well as the basic rights of citizens and, in federal countries such as the United States and Canada, the relationship between the central government and state, provincial, or territorial governments. Not all nation states have codified constitutions, though all such states have a jus commune, or law of the land, that may consist of a variety of imperative and consensual rules. These may include customary law, conventions, statutory law, judge-made law, or international rules and norms. Constitutional law deals with the fundamental principles by which the government exercises its authority. In some instances, these principles grant specific powers to the government, such as the power to tax and spend for the welfare of the population. Other times, constitutional principles act to place limits on what the government can do, such as prohibiting the arrest of an individual without sufficient cause. In most nations, such as the United States, India, and Singapore, constitutional law is based on the text of a document ratified at the time the nation came into being. Other constitutions, notably that of the United Kingdom, rely heavily on unwritten rules known as constitutional conventions.

Constitutional law refers to rights carved out in the federal and state constitutions. The majority of this body of law has developed from state and federal supreme court rulings, which interpret their respective constitutions and ensure that the laws passed by the legislature do not violate constitutional limits. Most constitutional legal issues 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. Many state constitutions also establish additional rights, but they may not take away any federal rights. 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.