Common law is law developed by judges through decisions of courts and similar tribunals (also called case law), rather than through legislative statutes or executive branch action. A "common law system" is a legal system that gives great precedential weight to common law, on the principle that it is unfair to treat similar facts differently on different occasions. The body of precedent is called "common law" and it binds future decisions. In cases where the parties disagree on what the law is, an idealized common law court looks to past precedential decisions of relevant courts. If a similar dispute has been resolved in the past, the court is bound to follow the reasoning used in the prior decision (this principle is known as stare decisis). If, however, the court finds that the current dispute is fundamentally distinct from all previous cases (called a "matter of first impression"), judges have the authority and duty to make law by creating precedent. Thereafter, the new decision becomes precedent, and will bind future courts.

In practice, common law systems are considerably more complicated than the idealized system described above. The decisions of a court are binding only in a particular jurisdiction, and even within a given jurisdiction, some courts have more power than others. For example, in most jurisdictions, decisions by appellate courts are binding on lower courts in the same jurisdiction and on future decisions of the same appellate court, but decisions of lower courts are only non-binding persuasive authority. Interactions between common law, constitutional law, statutory law and regulatory law also give rise to considerable complexity. However stare decisis, the principle that similar cases should be decided according to consistent principled rules so that they will reach similar results, lies at the heart of all common law systems.

Common law legal systems are in widespread use, particularly in England where it originated in the Middle Ages, and in nations that trace their legal heritage to England as former colonies of the British Empire, including the United States, Singapore, Pakistan, India, Ghana, Cameroon, Canada, Ireland, New Zealand, South Africa, Hong Kong and Australia.