Case law is the body of available writings explaining the verdicts in a case. Case law is most often created by judges in their rulings, when they write their decisions and give the reasoning behind them, as well as citing precedents in other cases and statutes that had a bearing on their decision. A single case may generate virtually no written interpretations or opinions, or, as is the case with many that come before the Supreme Court, it may generate a number of opinions as it works its way through various lower-circuit courts. These collected opinions can be referred to in the future by other judges when they make their rulings on similar cases, allowing the law to remain relatively consistent.

State trial courts, such as the California Circuit Court, do not publish opinions, and so do not generally add to the body of case law. Federal trial courts, such as the U.S. District Court, rarely publish opinions. The bulk of published opinions, available both online and in print form, come from both the state and federal higher courts. Official government agencies publish both federal Supreme Court and state higher court decisions, while the opinions of the U.S. Circuit Courts are published by private agencies.

Case law is often referred to as common law in many regions of the world, and is also known as judge-made law. This latter term derives from the fact that, while legislation is technically passed in most countries by a separate legislative branch, courts are able to exercise a moderate amount of quasi-legislative power through the use of precedent and case law. Case law is viewed by most people as a crucial part of a functioning judiciary, as it allows for courts to transform decisions that may have taken a great deal of time and energy to arrive at into a sort of de facto law, making future cases much easier to decide.

In some cases, a judge may intentionally go against established case law in an effort to begin the process of re-examining a precedent and perhaps ultimately changing it. This often happens with precedents in case law that a judge may consider out-dated or irrelevant in the contemporary climate. By issuing a decision that the judge knows will be appealed, he or she pushes the case into higher courts where the old established precedents may be overturned in favor of a new outlook.