Unreported decisions are judicial opinions that have not been published in any official or near-official case reporter. They are also called "unpublished decisions," "unreported opinions" and "unpublished opinions." They can be easy or difficult to find.
U.S. Court of Appeals: Unreported decisions of the U.S. Court of Appeals are published in West's Federal Appendix. They are also available in the Federal cases databases on Westlaw and Lexis. If you know the case name or number, unreported Court of Appeals decisions are also available through PACER.
U.S. District Courts: Unreported decisions of the U.S. District Courts can be found in Westlaw's District Court Cases -- Unreported (DCTU) database, as well as the database covering all District Court cases (DCT). Lexis includes unpublished district court decisions with its regular district court case files. If you know the case name or number, and have a password, you can also get an unreported district court decisions from PACER.
Lexis publishes all unreported USDC opinions starting June 21, 2005; prior coverage was selective. To eliminate unpublished opinions from a search, use FOCUS and add "and not notice(unpublish! or publish! or precedent!)" to the end of your search string.
U.S. State Cases: Unreported decisions from U.S. state courts can often be found on Westlaw or Lexis. Unreported state cases may also have appeared in a local legal newspaper, and they are generally included in case law CD-ROMs covering states where unreported opinions are considered persuasive. In some states, unreported decisions are available through the court's online docket system. (For more information on these materials, see the entries for the individual state.)
In addition, an unreported opinion from a high-profile case may be included in the Lexis GENFED;EXTRA or STATES;EXTRA files. And legally significant unreported opinions might have been published in a Mealey's Litigation Report, Westlaw Journal (formerly Andrews Litigation Reporter), CCH or other looseleaf reporters covering the topic in question.
Finally, you should be able to get the opinion from the relevant court clerk, the court library and/or the state library. If you're really, really lucky the opinion could be posted on the court's Web site.
Depublished Opinions: Apparently California and Oregon allow their "state supreme courts, on their own motion, to 'depublish' intermediate appellate court decisions" -- that is, the can change reported opinions into unreported opinions. This is practice is discussed in Aaron S. Bayer's "Unublished Appellate Decisions Are Still Commonplace" from the National Law Journal (August 24, 2009), in print or on Law.com. The article cites to Cal. R. Ct. 8.1125 and Ariz. R. Civ. App. P. 28(f).
Frequently Asked Questions: Joseph Gerken's "A Librarian's Guide to Unpublished Judicial Opinions" asks and answers most of the common questions about unreported decisions (96(3) Law Library Journal 475 (Summer 2004)). Questions addressed include: Why Do Courts Issue Unpublished Opinions? How Can a Court Say That a Decision is Not Precedent? Is Depublication the Same as Nonpublication? Can an Unpublished Decision be "Used" or "Cited"? Is it Possible to Use an Unpublished Decision in a Circuit With a Do Not Cite Rules? and Can an Attorney Ethically Ignore an Unpublished Decision That Supports a Client's Case to Avoid Sanctions for Violating a Do Not Cite Rule? See also Aaron S. Bayer's "Unublished Appellate Decisions Are Still Commonplace" from the National Law Journal (August 24, 2009).
Precedential value: The precedential value of unreported/unpublished opinions issued by Federal appellate courts is established in the court rules for each Circuit (see "Federal Court Rules"). For a discussion of these rules, see "From Anastasoff to Hart to West's Federal Appendix: The Ground Shifts Under Un-Citation Rules," 4 Journal of Appellate Practice and Procedure 1 (Spring 2002) and "Unpublished Appellate Decisions Are Still Commonplace" by Aaron S. Bayer, National Law Journal (August 24, 2009).