Law is a system of rules that governs the behaviour of people and is enforced by a controlling authority. It shapes politics, economics, history and society in a wide variety of ways. A law can be written or tacit, such as a constitution, and it can be influenced by a range of social factors including custom and practice, laws of nature, religious beliefs, ethics and human rights.

Most countries employ a system of law to govern their societies and enforce their rights. Some, such as the United States, have a common law system that relies on case law (judge decisions) rather than on statutes. Others, like Japan, have a civil law system that relies on codes of legal rules to guide judges in their decision making. A person who advises and advocates on the law is called a lawyer, barrister or solicitor.

The primary purposes of the law are establishing standards, maintaining order, resolving disputes and protecting liberties and property. It is important for all citizens to have access to the law, which is often provided by a constitution, and for it to be enforceable against people who do not follow its principles. In addition, a stable legal system is important for trade and foreign relations.

A legal system typically has several layers, ranging from constitutional and civil rights to criminal and administrative law. Constitutional law outlines the basic principles that a country is expected to uphold, such as freedom of speech and religion. Civil rights include the right to equal protection under the law and the right to due process. Criminal law, on the other hand, deals with crimes such as murder and treason that are considered to be against the state.

Other laws include those dealing with family matters such as divorce and child custody, financial issues such as bankruptcies, and employment law. Laws concerning medical treatment, such as physician-patient privilege and medical malpractice, are also an important part of any legal system. Laws regulating the use of genetically modified organisms and their effects on human health are a growing area of study in the field of biolaw. International law focuses on the way that countries interact with each other, and it can include treaties such as the Geneva Conventions and the Roerich Pact on military action. It can also deal with environmental law and the law of war.