In legal research, Shepard's Citations is a citator, a list of all the authorities citing a particular case, statute, or other legal authority. The verb Shepardizing refers to the process of consulting Shepard's to see if a case has been overturned, reaffirmed, questioned, or cited by later cases. Although the name is trademarked, it is also used informally by legal professionals to describe citators in general—for example, Westlaw's similar tool called Key Cite.
The name derives from a legal service begun by Frank Shepard (1848–1902) in 1873, when Shepard began publishing these lists in a series of books indexed to different jurisdictions. Initially, the product was called Shepard’s Adhesive Annotations. The citations were printed on gummed, perforated sheets, which could be divided and pasted onto pages of case law. Known as “stickers,” these were literally torn to bits and stuck to pertinent margins of case reporters.
By the early 20th century, the Frank Shepard Company was binding the citations into maroon volumes with Shepard’s Citations stamped in gold on their spines, much like the ones still found on library shelves.
Under the leadership of William Guthrie Packard, the company endured the Great Depression and continued to grow. It moved to Colorado Springs in 1948; in 1951, it adopted the name Shepard's Citations, Inc.
In 1996, Shepard's was purchased by LexisNexis (a subsidiary of Reed Elsevier since 1994). After this acquisition, LexisNexis engaged in a "multi-million-dollar Citations Redesign (CR) project" which "redesigned the way we process case law and citations." 
In March 1999, LexisNexis released an online version of the Shepard's Citation Service. While print versions of Shepard's remain in use, their use is declining. Although learning to Shepardize in print was once a rite of passage for all first-year law students, the Shepard's Citations booklets in hard copy format are extremely cryptic compared to the online version, because of the need to cram as much information about as many cases in as little space as possible.
Shepard's paper format consists of long tables of citations (with full case titles omitted) preceded by one or two-letter codes indicating their relationship to the case being Shepardized. Before Lexis became widely available, generations of lawyers (and law clerks and assistants) had to manually locate the Shepard's entry for a case, decipher all the cryptic abbreviations, then manually retrieve all the cases which were marked by Shepard's as criticizing or overruling a particular case, to determine whether the later cases had directly overruled that particular case on the specific issue for which one was interested. In many jurisdictions in the U.S., it is still possible to cite a case as good law even though it has been overruled, as long as it was overruled on another issue and not the specific issue for which it is being cited.
In 2004, market research by LexisNexis indicated that most attorneys and librarians conduct the majority of their research online, but "that there are a number of experienced attorneys, principally in smaller firms, who still prefer print and who are extremely unlikely to change their ways."
The company representative added that:
Given the ripe old ages at which many lawyers continue to practice their profession, we don’t see the market for Shepard’s in print disappearing any time soon. Clearly, subscription lists for Shepard’s products are declining as online usage grows. Attrition has been steepest in large law firms, where relatively junior associates conduct a great deal of citations research online. Attrition has been less steep in libraries and small firms where attorneys who prefer print continue to do their research. For many years, attrition in academic law libraries was relatively low. Many law school libraries continued to retain relatively substantial collections of Shepard’s in print. In recent years, attrition has increased—especially in law schools that no longer teach their students how to Shepardize in print. But because many law school libraries are open to the public (or at least to graduates of the school), including practicing attorneys in the communities they serve, a typical law school library continues to retain at least a basic collection of Shepard’s print products.
After Shepard’s became a part of LexisNexis, we totally redesigned the way we process case law and citations. The multi-million-dollar Citations Redesign (CR) project was intended to eliminate duplication and allow us to deliver current, accurate information unmatched by our competition. The ability to produce Shepard’s print pages quickly and efficiently was built into the CR requirements—another factor contributing to the continuing viability of Shepard’s in print.
Today, LexisNexis users can Shepardize citations online; all cases displayed on LexisNexis bear a "Shepardize" link in their header and nearly always show an icon in the upper left corner of the Web page indicating the status of the case as citable authority. The icon itself, when clicked, brings up a full Shepard's report for the case.
The report indicates exactly how later cases cited the case being Shepardized with plain English phrases like "followed by" or "overruled" rather than by using the old abbreviations. Additionally, the report shows the full case title (that is, the names of the plaintiff and defendant) and full citation for each of the later cases. This is important because lawyers can usually distinguish criminal from civil cases by looking at the title. Criminal cases (with the exception of habeas corpus cases) are always titled "People v. [defendant]" or "State v. [defendant]." Often, a criminal case may cite a civil case for a point of law which a civil litigator does not care about, and vice versa.
Finally, the online report has the convenience of allowing the user to simply click on the hyperlink for any listed case to retrieve it instantly (if it is within the user's access plan), whereas users of Shepard's print version had to dash through long law library aisles to retrieve heavy legal reporter volumes, one for each case (and then someone had to put all those volumes back).