If a court has already decided a similar legal issue, does that earlier decision control a new case? Case law is the body of law developed through judicial opinions and decisions. Those decisions explain how courts interpret and apply constitutions, statutes, regulations, and earlier decisions to particular facts.

Finding a similar opinion is only the beginning. Its weight depends on which court decided it, the jurisdiction, the issue actually resolved, its precedential status, later treatment, and how closely the material facts match.

Case law compared with statutes and regulations

Different forms of law come from different institutions, but they operate together:

The Library of Congress case-law guide describes case law as law developed from judicial opinions over time. A case may interpret a statute or regulation, decide whether government action is constitutional, or apply an existing rule to new facts. It is therefore misleading to treat case law as law that exists wholly apart from written legal texts.

How precedent and stare decisis work

Precedent is an earlier decision used as authority in a later matter. *Stare decisis* describes the practice of following prior decisions when the same legal point arises again. It supports consistency, but it does not make every sentence in every opinion permanently binding.

Under vertical stare decisis, a lower court follows controlling decisions of higher courts in the same hierarchy. Under horizontal stare decisis, a court considers its own prior decisions. The exact rule can vary by court, and a court may distinguish an earlier decision when the legally important facts or issue differ.

Researchers must also identify what the court actually decided. The holding—the rule necessary to resolve the issue before the court—carries different weight from background discussion, a hypothetical, or an observation unnecessary to the result. Separate concurring and dissenting opinions can illuminate arguments, but they do not become the controlling opinion merely because they are persuasive.

Binding versus persuasive case law

Binding authority is precedent the court must follow on the relevant issue. Persuasive authority may influence the court but does not compel the result. The label depends on the court receiving the argument, not just the reputation of the court that issued the opinion.

For example, a published precedential decision of a federal court of appeals generally binds federal district courts within that circuit. The U.S. Courts explains that most courts-of-appeals decisions are final and bind lower courts in the same circuit. A decision from another circuit may be persuasive, especially on an unresolved federal question, but it is not controlling merely because the issue is similar. Publication and local circuit rules still matter.

State publication rules provide another filter. In California, Rule of Court 8.1115 generally bars citation of unpublished Court of Appeal and superior court appellate-division opinions in other actions, subject to specified exceptions. Published opinions may be cited once certified or ordered published. That California rule was checked through July 31, 2026; it should not be applied to federal courts or another state.

How to read a case citation

A citation is an address for a decision. The Library of Congress uses this example in its case-citation guide:

`Stearns v. Ticketmaster Corp., 655 F.3d 1013 (9th Cir. 2011)`

• `Stearns v. Ticketmaster Corp.` identifies the lead parties.
• `655` is the reporter volume.
• `F.3d` abbreviates the Federal Reporter, Third Series.
• `1013` is the page where the opinion begins.
• `(9th Cir. 2011)` identifies the deciding court and year.

A pinpoint citation may add the exact page supporting a proposition. Citation formats vary by court and document, so this anatomy helps locate an opinion but is not a complete citation manual.

A five-step case-law research workflow

1. Define the issue and jurisdiction. Identify the precise legal question, court system, location, and procedural posture. A state contract issue and a federal constitutional issue do not start with the same authority set.
2. Find candidate decisions. Search with legal concepts, relevant statutory language, court, and fact terms. Secondary sources can suggest cases, but read the opinion itself.
3. Rank the authority. Check the deciding court, hierarchy, publication or precedential status, and whether the opinion addresses the same issue. Start with controlling higher-court authority.
4. Validate current status. Review later history and treatment to see whether the decision was reversed, vacated, limited, questioned, distinguished, or affected by later law.
5. Compare the holding and facts. State the rule the court needed to decide, then compare the legally material facts. A keyword match or similar outcome is not enough.

The California Courts research guide similarly tells researchers to consider facts, deciding court, date, whether the case remains good law, and whether the court must follow it.

How to check whether a case is still good law

“Good law” means the decision remains usable authority for the proposition being asserted. Check the case's direct appellate history, then review later decisions that discuss it. A case can remain valid on one point while another proposition has been rejected or superseded.

Also confirm publication and precedential status, current court rules, and whether a later constitution, statute, regulation, or controlling case changed the governing law. A citator can organize treatment signals, but the signal is a starting point: read the later decision and verify what happened to the precise proposition. A public law library can help a researcher identify appropriate current tools.

Common case-law research mistakes

• Treating the first search result or headnote as a substitute for the court's opinion.
• Using a case from the wrong jurisdiction without explaining why it is persuasive.
• Calling an opinion binding without checking court level and precedential status.
• Relying on similar facts while ignoring a different legal issue or procedural posture.
• Quoting an old decision without checking later history, treatment, and intervening law.
• Assuming an unpublished or nonprecedential disposition can be cited under every court's rules.

What is a simple example of case law?

If an appellate court interprets a statutory phrase and applies that interpretation to the dispute, its opinion becomes case law. Whether a later court must follow it depends on hierarchy, jurisdiction, and precedential status.

Is case law the same as common law?

Not exactly. Case law broadly includes judicial decisions interpreting constitutions, statutes, regulations, and prior decisions. Common law usually refers more specifically to doctrines developed through judicial decisions rather than enacted text, although usage varies by context.

Can case law override a statute?

A court can interpret a statute and may hold it unconstitutional, but a lower court cannot disregard valid controlling statutory text merely because another case seems preferable. The answer depends on the court's authority, the holding, later law, and the legal issue presented.

Case law becomes useful when a researcher can explain not only what an opinion says, but why that court's decision carries weight for the present issue. Always validate hierarchy, precedential status, current treatment, and material facts before relying on a case.