The alt.* hierarchy is one of the major class of newsgroups in Usenet, containing all newsgroups whose name begins with "alt.," organized hierarchically.
Thr alt hierarchy is not confined to newsgroups of any specific subject or type, although in practice more "formal" groups tend not to occur in alt.
Unlike most other hierarchies there is no centralised control of the hierarchy and anyone who is technically capable of creating a newsgroup can do so. However in practise most newsgroups follow an informal procedure involving a public discussion in alt.config before being created, this procedure is designed to help the potential creator better understand what factors contribute to a newsgroups success.
It is up to each individual news administrator if to add a new newsgroup, and some will not do so if the group has not been discussed in alt.config. As a result groups that do not follow this procedure are usually not well-propagated.
News group removal in theory occurs in much the same way as newsgroup creation, however as a matter of practice most news administrators do not remove newsgroups.
The birth of the alt.* hierarchy is tied to a drastic transformation of the Usenet: the Great Renaming in 1987. The "backbone carriers," or the backbone cabal as they have been referred to by some users of the Usenet, were vital hubs in the distribution chain of most of the newsgroup postings. Their effort to change the way newsgroups are organized led to objections from some vocal Usenet users.
In particular, the creation of the talk.* hierarchy for discussions of controversial or sensitive issues by the Renaming did not go well. As an alternative, the alt.* hierarchy was suggested by one of the cabal members, Brian Reid. It was a network without the backbones, thereby free from backbones' influences on creating or not creating a new newsgroup. Some suggest that Brian Reid was also interested in starting a recipe group outside the major hierarchies. ,  The first newsgroup on alt hierarchy was his alt.gourmand.
The name alt is said to humorously refer to "anarchists, lunatics, and terrorists," but understood today as "alternative," by most people.
Alt has since become home for a wide variety of things that did not fit elsewhere. In particular, there are many alt.fan newsgroups, mostly devoted to discussions of the work and life of famous people: writers, musicians, and athletes have alt.fan groups. This sub-hierarchy has also been used for self-promotion by otherwise unknown people, and during a notorious trial, alt.fan.karla-homolka was created to get around the Canadian news blackout on the case.
Two major sections of the alt.* hierarchy, the alt.sex.* and alt.binaries.* hierarchies, have been found to fit better in the alt.* hierarchy than the Big Eight. Because of the inevitably lurid and sometimes offensive subjects that it would cover, newsgroup administrators objected to the inclusion of one or more newsgroups covering sexual topics in the Big Eight (including the existing rec.arts.erotica), fearing that they may prevent the major news hierarchies from being widely distributed. News administrators are free to add any or all of the alt.sex.* newsgroups without having to worry about conflicting with the Big Eight. Likewise, any and all of the alt.binaries.* newsgroups can be accepted or rejected by administrators if they choose.
Several extensions of the alt.* hierarchy have become quite successful on their own. A number of newsgroups have taken advantage of the freedom of the alt.* hierarchy to create a number of newsgroups that specialize on certain topics, as opposed to the broader "generic" discussions of the Big Eight hierarchy. For instance, the rec.* hierarchy may be home to the movie discussion newsgroups rec.arts.movies.current-films, rec.arts.movies.past-films, and rec.arts.movies.reviews; but the alt.movies.* hierarchy contains more focused discussion groups including alt.movies.silent, alt.movies.hitchcock, alt.movies.kubrick, and alt.movies.visual-effects.