Law is a body of rules that society or government develops to deal with issues such as crime, business agreements and social relationships. It may refer to a specific field of law, such as criminal, civil or space laws, or it can be used more broadly to refer to the system of legal rules and the people who study and work in it.

The precise nature of law is a subject of longstanding philosophical debate. Ancient Greek philosophers such as Plato and Aristotle theorised about it largely from the perspective of a legislative and court-based political system. This reflected the reality of law in their societies, in which the majority of legal disputes were resolved by legislators and judges based on precedent.

But the modern concept of law is very different from that which influenced these theorists. For one thing, in advanced legal systems such as those of the Anglophone world, special legally trained lawyers represent individuals in their dealings with the state and courts. And judicial power to determine the law has expanded dramatically over time, in ways that pre-modern thinkers such as Locke and Montesquieu could not have foreseen.

This has meant that the fields of law have diversified, reflecting an increasing diversity of human activities. Space law, for example, deals with the relations of countries with each other and the universe beyond Earth; banking law encompasses regulation on how much capital banks must hold and best practice for investment; and competition law, which traces its roots back to Roman decrees against price fixing and English restraint of trade doctrine, is concerned with businesses that try to distort market prices at the expense of consumer welfare.

Other areas of law include labour law, which covers the tripartite industrial relationship between worker, employer and union; inheritance law; family law; and medical jurisprudence. Law also applies to the process of adjudication itself, with the laws of evidence and of procedure determining which materials are admissible in court for a case to be heard.

Historically, some of the most detailed legal systems have been religious in nature, with Jewish Halakha and Islamic Sharia being two of the most prominent examples. This has often led to a great deal of elaboration, with the religious scriptures acted on through interpretation, Qiyas (reasoning by analogy), Ijma (consensus) and legal precedent to produce a comprehensive body of law.

This broad range of areas of law can be grouped into three main categories for convenience, though the subjects inevitably overlap and intertwine. Those that apply to an individual are called civil laws; those that apply to a group of citizens or the state itself are called criminal laws. A third category, administrative law, covers the way in which governments regulate their own activities. It encompasses everything from the standards that must be met by airlines for safety and security to rules governing the use of the internet. It also includes the rules that govern how governments conduct themselves internationally, such as treaty negotiations and foreign policy.