Originally, it merely provided lists of the names of notable people, for example all MPs or all bishops. Starting with the 1897 edition, it listed people alphabetically and provided fuller biographical details.
A full online edition of the work was launched in 2005. However, it continues to be published annually in hard copy.
A history of Who's Who was published to coincide with the 150th edition in 1998.
Subjects include peers, MPs, judges, very senior civil servants, and distinguished writers, actors, lawyers, scientists, researchers, and artists. Some (such as those holding a Professorial Chair at Oxford or Cambridge) are included automatically by virtue of their office; those in less hierarchical occupations are included at the discretion of the editors.
Inclusion in Who's Who, unlike many other publications, has never involved any payment by the subject, or even any obligation to buy a copy. Inclusion has always been strictly regulated by prominence in public life or professional achievement. Inclusion has therefore come to carry a considerable level of prestige. The Wall Street Journal has said that an entry in Who's Who "really puts the stamp on eminence on a modern British life", and the Daily Mail has described it as "Britain's most famous reference book".
Once someone is included in Who's Who he or she remains in it for life, so for example MPs are not removed when they leave Parliament. The 7th Earl of Lucan is still listed in the book, even though he has been missing since 1974 and was declared legally dead in 1999.
Who's Who has been criticised for its conservatism. For example, all members of the English, Scottish, British and United Kingdom Peerage and Baronetage are included, however minor their achievements, but many better-known people are not. Since subjects remain in the volume until death, the editors tend to prefer establishment figures to those (such as pop stars and sportsmen) who enjoy greater celebrity for a time, but may then slip out of public view entirely. However, the editors now include more such people than before but the numbers are a small proportion of the total. Jeremy Paxman has pointed out that the publication is dominated by people who are active in British public life, including the members of the Scottish Parliament, Welsh and Northern Ireland Assemblies, as well as Members of the House of Commons, the Chief Executives of all UK cities and counties, and foreign Ambassadors accredited to London. He also points out that there is a high proportion of Oxford and Cambridge graduates among the new entrants.
Occasional problems arise with the publication's reliability as a reference source because the entries are compiled from questionnaires returned to the publisher by the featured subjects. Some checks are made by the editors but subjects may omit anything they wish and such errors of omission can be difficult to identify. Examples that have been spotted include: the playwright John Osborne did not acknowledge an estranged daughter in his entry, Carole Jordan does not mention any marriage in her article, although her ex-husband, Richard Peckover, does in his. Paxman has also calculated that only 8% of new entrants in 2008 make any reference to marital breakdown, which is far below the national average.
On the other hand, having entries compiled by the subject allows information to be included which would not otherwise be a matter of readily-accessible public record. It also allows the entries to convey something of the character and interests of the subject, and not just his or her professional achievements, especially in the description of "recreations". From conventional references to fishing, reading or opera (which still feature prominently), listed recreations now include "Maintaining rusty old cars", "Walking my iPod", "Anglophobia", "Being first in Who's who" and "Contemplating revenge".
Sometimes, there is an excess of detail: the prolific romantic novelist Dame Barbara Cartland listed each of her publications, many hundreds of books, in hers, together with a list of her other achievements; the result was one of the longest entries in the history of Who's Who.
After a person's death, entries are preserved in the volumes of Who Was Who. They are usually as they appeared in the last Who's Who before the death, with the date of death appended. The first volume covered deaths in 1897–1915, but more recently they have appeared at ten-year intervals, and now appear every five years.
The name has been widely copied, and now there are many publications with "Who's Who" in the title, though they are not from the same publisher. One of those is Who's Who in Scotland, (2007 edition: ISBN 978-0-9546631-5-5), an annual publication first published in 1986. It is published by Carrick Media, who are based in Ayr.