Case law, also known as common law, is a type of law that is based on judicial decisions from previous cases, rather than law based on constitutions, statutes, or regulations. It uses the detailed facts of a legal case that have been resolved by courts or similar tribunals, and these past decisions are called "case law" or precedent. The principle of stare decisis, meaning "let the decision stand," is the principle by which judges are bound to such past decisions, drawing on established judicial authority to formulate their positions.

Case law differs from statutory law, which are codes enacted by legislative bodies, and regulatory law, which are established by executive agencies based on statutes. In some jurisdictions, case law can be applied to ongoing adjudication, such as criminal proceedings or family law.

The different roles of case law in civil and common law traditions create differences in the way that courts render decisions. Common law courts generally explain in detail the legal rationale behind their decisions, with citations of both legislation and previous relevant judgments, and often interpret the wider legal principles. The necessary analysis, called ratio decidendi, then constitutes a precedent binding on other courts.

Key points to note about case law include:

• Case law is law that is based on judicial decisions rather than law based on constitutions, statutes, or regulations.
• Case law concerns unique disputes resolved by courts using the concrete facts of a case.
• Statutes and regulations are written abstractly, while case law uses the detailed facts of a legal case that have been resolved by courts or similar tribunals.
• Case law refers to the collection of precedents and authority set by previous judicial decisions on a particular issue or topic.
• Case law differs from one jurisdiction to another, and a case in one jurisdiction would not be decided using case law from another jurisdiction.
• Federalism plays a major role in determining the authority of case law in a particular court.
• Case law, like legislation, can change over time, and a lawyer needs to be able to check whether it has been subject to subsequent judicial consideration and whether it remains good law.

Overall, case law is an important source of law that helps to shape legal decisions and interpretations.