National law is the set of laws that govern a country or State. In the United States, for example, the laws that make up national law are those passed by Congress and signed into law by the President. These laws could be constitutional statutes, executive orders or judicial decisions. In some countries, national laws also include those made by international agreements. In this case, a country may decide to internationallyize an agreement by ratifying it and making the treaty rules part of its national law.

National laws are often based on traditional systems of law and government. For example, in countries with a common law tradition, many national laws come from judicial decisions. In some countries, such as the United Kingdom, laws are created by enactment. In other cases, laws are based on the conventions of the particular field in which the law is being created. For example, a law that regulates the environment would be created according to the conventions of the environmental field such as the United Nations Framework Convention on Climate Change (UNFCCC).

Other national laws are based on a written constitution or a number of basic laws that establish the principles guiding a government. For instance, most countries have a constitution that sets out the checks and balances and limitations of governmental authority. Many of these systems have a Bill of Rights that is not always justiciable but is considered to be the highest standard a government should attain. Other systems incorporate international human rights norms into their system of law by basing national court decision on international and regional law or using it as an aid to interpret domestic laws. This approach is called legal harmonization.