Over the years, Wikipedia has made use of a wide range of classification systems, to categorize existing topics, normalize the namespace by identifying and merging similar articles, and identify interesting topics not yet written about (by drawing on external lists or catalogs).

Early on, some articles consisted entirely of lists of entries, each of which started out as a red link, or as a blue link to another article. Soon, these lists began to be listed — organized into higher-order lists of lists and lists of lists of lists. Two types of lists emerged on Wikipedia determined by their scope: item lists and topics lists...

Item lists

Most lists on Wikipedia are item lists that present the members of a class of things. Examples of item lists include List of cat breeds, List of highest-grossing films, and List of presidents of the United States.

Topics lists

Topics lists present the subtopics belonging to a particular subject. From the very beginning of Wikipedia, editors have been organizing its contents, by subject, using topics lists. There are two formats of stand-alone topics lists on Wikipedia: alphabetical indexes, and hierarchical outlines.

Indexes

Indexes attempt to list all the topics on a subject in alphabetical order. Examples include Index of robotics articles, Index of ancient Egypt–related articles, and Index of underwater diving. For more information, see Wikipedia:Indexes.

Outlines

Outlines list the articles belonging to a subject as a tree structure arranged by subtopic. Each outline serves as a table of contents for a subject's coverage on Wikipedia. Some examples of outlines are Outline of forestry, Outline of Google, and Outline of underwater diving. There is even an Outline of Wikipedia, that organizes Wikipedia's articles about itself. For more information, see Wikipedia:Outlines.

Lists of missing articles

Another historical milestone was the application of topics lists for tracking missing articles -- tens of thousands of topics that were either frequently linked to in other articles (see Special:WantedPages, Wikipedia:Requested articles, and Wikipedia:Most-wanted articles), or were present in other catalogs and indices, but that had yet to have articles about them on Wikipedia.

These lists were compiled by curators keen on identifying what was missing, to alert editors on what new articles to create. A feature of the MediaWiki software is that links to articles that don't exist yet appear red. Creating such an article was made very easy: all an editor had to do was click on its red link and start typing on a new page that MediaWiki instantly put before them for their convenience. In addition to assisting editors in expanding Wikipedia's coverage, the lists also proved helpful in addressing systemic bias by showing subjects lacking attention.

Cross-language efforts to identify lists of articles every Wikipedia should have (often roughly 100 or 1000 -- see m:List_of_articles_every_Wikipedia_should_have and m:Wiki99) started to develop early on as well.

A category system was added to MediaWiki ^when? (the software program that runs Wikipedia) as a way to tag pages with the classifications they belong to, which are displayed at the bottom of every article.

Style guides have tried to address naming categories, handling overlapping categories, and handling composite categories [should green scandinavian tigers be in all of green tigers, scandinavian tigers, and green scandinavian animals?].

Finding simple ways to query the intersection of two categories was a long-standing technical challenge. ^when?

The introduction of the category system, being redundant with Wikipedia's system of lists, inspired philosophical discussions as to when it was appropriate to have a list vs a category -- should lists only exist where the criteria for inclusion were clear, and it was possible in theory to complete the list? This debate continues to this day, and is a source of tension between editors who specialize in categories and those who support lists: which topics if any should have both a category and a list?

Many subjects have navigation boxes that list their major articles or subtopics. Such a topic-list-in-a-box often overlaps with a corresponding list article and/or category, either or both of which may be listed in the navbox.

The box itself is a mediawiki page, often in the Template namespace.

There are two main types of navboxes: navigation footers and navigation side bars.

Their own namespace, portals were a popular form of list for a time. Echoing a Web trend to develop portals for fansites and smaller, topical catalogs.

As interest and activity died down, these pages, which had been templated as sensitive to current events -- a mix of the Main Page and a timeless outline -- began to get stale, and many were deleted in a Portal purge.

List development by the Wikipedia community has been (and still is) far more extensive than first meets the eye. There are more than 30 list-based navigation systems (WP linkified classification systems) and list types on (or of) Wikipedia. Here are most of them:

1. Embedded lists
2. Stand-alone lists
3. Headings (displayed as article tables of contents)
4. Embedded outlines
5. Stand-alone outlines
6. Indices
7. Categories
8. Wikipedia:Vital articles
9. Wikipedia:Version 1.0 Editorial Team/Core topics
10. Navigation footers
11. Navigation sidebars
12. WP Books, deprecated but still available as a link system through the Wayback Machine, with a working category front-end (categories are fully functional there)
13. Special:AllPages
14. the A-Z index interface to All pages
15. Special:Prefixindex
16. Maintenance reports (they're lists)
17. Tables
18. Figurative system of human knowledge (from the Encyclopédie), linked to Wikipedia pages
19. Outline of academic disciplines
20. Outline of Knowledge (part of the Propædia of the 15th edition of Encyclopædia Britannica), linked to Wikipedia pages
21. Outline of the knowledge of humanity
22. Outline of Roget's Thesaurus, linked to Wikipedia pages
23. Fields of doctoral studies (United States), linked to Wikipedia pages
24. Joint Academic Classification of Subjects, linked to Wikipedia pages
25. Bliss bibliographic classification, linked to Wikipedia pages
26. Colon classification, linked to Wikipedia pages
27. Cutter Expansive Classification, linked to Wikipedia pages
28. List of Dewey Decimal classes, linked to Wikipedia pages
29. Library of Congress Classification, linked to Wikipedia pages
30. Universal Decimal Classification, linked to Wikipedia pages
31. WikiProject topics lists
32. the stub classification system
33. portal topics sections
34. output from the script SearchSuite

Historical article : Lists on Wikipedia Information

There should be a historical article on Lists_on_Wikipedia -- a topic widely discussed in the literature on encyclopedias, epistemology, and classification. This section is for a draft of it.

'Lists on Wikipedia are a substantive part of Wikipedia's index, ^{citation needed} and play a role in developing depth within a topic, covering less well-known subtopics or entities. ^{citation needed}

Lists and indexes have long been a staple of reference works, some essential ones being references unto themselves. ^{citation needed} The scope and diversity of these lists has expanded as references became digital, ^{citation needed} and Wikipedia has been one of the active promulgators of the list as a first-class article. ^{citation needed}

• List of lists of lists ^{citation needed}
• WP:List of unusual articles ^{citation needed}
• (check lists by readership ^{citation needed}, active edits ^{citation needed}, size ^{citation needed}, longevity ^{citation needed})
• (current-event lists ^{citation needed}, ancient-history lists ^{citation needed}, meta lists ^{citation needed}) - incl. WP:Outlines ^{citation needed}