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Case law are opinions issued by the judicial branch of government. They can be on statutory, regulatory, pure common law questions, or some combination of the three. Because the U.S. is a common law jurisdiction, cases can have precedential value based upon the principle of stare decisis.
Any given area of law may involve statutes, regulations, and/or case law. These three types of primary law exist on both the federal and state level. Figuring out what primary law is on point to your particular issue can be difficult, which is why it is important to look for secondary sources first that pull relevant authority from all three branches of government, on both a federal and state level.
Of the three kinds of primary law to research directly after surveying secondary sources, it is usually best to search for statutes and then regulations as statutes provide the authority for agencies to create regulations. Case law should usually come last as the courts are called upon to interpret both statutes and regulations. If your issue is purely a common law question, the only source of law will be case law.
By this point in your research, you may already have citations to cases from secondary sources or from researching statutory or regulatory law. If that is the case, you will want to look those cases up. Regardless, it is at this stage in the research that you will want to turn to case law resources to confirm that you have assembled all relevant citations.
There are many different resources for researching case law. For more information about basic case law research, see Tarlton's legal research guide, "Finding a Case." The resources highlighted in this guide include:
To make sure that the court opinions you find are still good law, you must use a citator, whether it is Shepard's from Lexis or KeyCite from Westlaw.