Who's Who (or "Who is Who") is the title of a number of reference publications, generally containing concise biographical information on a particular group of people. The title is also used as an expression meaning a collection or group of noted persons; as in the sentence, "The actors in the film were a Who's Who of the great American comedians of the time".
The title "Who's Who" is in the public domain, and thousands of Who's Who compilations of varying scope and quality have been published by various authors and publishers. The oldest and best known is the annual British publication Who's Who, a reference work on contemporary prominent people. Another reputable and long standing publication is Canadian Who's Who, which has published biographies of prominent Canadians based on merit alone since 1910.
However, many publications using the title are vanity publications, where the inclusion criterion is the biographee's willingness to buy the book, with the business model consisting of selling books directly to the biographees.
- Who's Who, a listing of prominent British people since 1849; people who have died since 1897 are listed in Who Was Who
- Marquis Who's Who, a series of books published by Marquis, primarily listing prominent American people but including Who's Who in the World.
- Canadian Who's Who, a listing of prominent Canadians since 1910
- Who's Who in Australia, a listing of prominent Australians since 1923
- Who's Who in American Art, a listing of prominent American artists
- Kraks Blå Bog, Denmark's Who's Who
- Who's Who in Europe, a listing of prominent Europeans since 1965
- (French) Who's Who in France, a listing of prominent French or people living in France since 1953
- Swiss Who's Who, a biographical dictionary of prominent Swiss or people living in Switzerland
- Who's Who in British History
- Who's Who in Scotland, a listing of prominent Scots since 1986
- Who's Who, by Metron Publications, a listing of prominent Greeks since 1992
- Who's Who in the DC Universe a listing of DC Comics characters
- Who's Who Among American High School Students listing what it deems to be American high school and college students who particularly excel in the realm of academic achievement. The publishing company closed in 2007.
- Who's Who Among Students in American Universities & Colleges, published by Randall-Reilly Publishing Co., LLC is a list of high achieving college students nominated by the college faculty.
- Who's Who in American Real Estate, a resource tool of professional real estate agents in America since 1983.
- Who's Who of Southern Africa, a resource tool of noteworthy people in Southern Africa, and a professional networking platform. It is not the same as the official print publication, but a website which anyone may join and create their own profile.
- Hinterland Who's Who, a series of 60-second public service ads profiling Canadian animals and birds, produced by Environment Canada in the 1960s
- International Who's Who by Europa Publications, a Taylor and Francis imprint
- Who's Who in the CIA, a book published in East Berlin in 1968 with the assistance of the KGB and the HVA purporting to reveal the identities of thousands of CIA officers.
- Who's Who International SA, Lausanne, Switzerland created in 1958.
- Hübners Who Is Who , by Who is Who, Verlag für Personenenzyklopädien AG, Zürich, which has a number of national editions throughout Europe:
- one common edition for some German-speaking countries (Austria, Germany and Luxembourg): Who is who in dem deutschen Sprachgebiet, as well as the separate national editions for
- Poland: Who is Who w Polsce
- Czech Republic: Who is ...? (v České republice)
- Slovakia: Who is Who v Slovenskej republike
- Hungary: Who is Who Magyarországon
- Romania: Enciclopedia Personalităţilor din România: Who is Who
- Greece and Cyprus: Who is Who στην Ελλάδα
- Turkey: Türkiye’de Who is Who
- Russia (including Belarus, and Russians from the other countries of the former USSR): Who is Who в России
- Vietnam: Ai là ai
- Yugoslavia: Ko je ko u Jugoslaviji (1970)
Some Who's Who books have a title in the language of the country concerned:
- Croatian: Tko je tko u Hrvatskoj, bilingual edition (1993)
- Danish: Kraks Blå Bog (since 1910) annually
- German: Wer ist's? (1905–1935) and Wer ist wer? (since 1951) almost annually
- German: for East Germany: Wer war wer in der DDR? 
- Norwegian: Hvem er Hvem? (since 1912) 14 editions in the 20th century
- South Africa: Who's Who of Southern Africa (since 1910) annually
- Spanish: ¿Quién es quién?
- Swedish: Vem är det (since 1912) every second year
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