How many editions of Dungeons & Dragons are there?
As a generic answer for the non-hardcore fan, I think most people would define the following:
- OD&D (aka D&D 0e) including the original Brown Box and White Box versions from 1974.
- Classic (BECMI) D&D (including everything from BD&D in 1977 through the Rules Cyclopedia of 1991 and the Classic D&D Game starter set)
- AD&D 1st Edition (core books released 1977-1979)
- AD&D 2nd Edition (released 1989)
- D&D 3rd Edition (2000)
- D&D 3.5 Edition (revision of 3rd edition, released 2003)
- D&D 4th Edition (2008, including D&D Essentials, as D&D 4th edition has been erratad to the point that Essentials is at now)
- D&D 5th Edition (2014, aka D&D Next)
Certainly you could split it further (the various Mentzer/Moldvay differences, etc.) , but those are the major divides most gamers would likely note.
Of course, no edition of D&D survives contact with a gaming group intact; if there are 40,000 D&D sessions this weekend, there will be just about as many “versions” being played.
- Chainmail Fantasy Supplement in 1971
- Original Dungeons & Dragons published in 1974
Note that the basic game uses d6's for almost everything, and is written to use Chainmail as the combat system, rather than the "alternate combat system," which would later become the standard D&D combat mechanic. Further, only three classes exist - Fighting Man, Cleric, Magic-User — and neither multi-classing nor demihumans work the same as later.
- Supplement I: Greyhawk published in 1975
The added rules made Dungeons & Dragon into a form we recognize today. It changes to the HD mechanic that is used in every later edition, added thieves and the thief skills, changed multiclassing towards what would be used in AD&D
- Basic Dungeons & Dragons by Holmes in 1977
- Advanced Dungeons & Dragons in 1977-1979
- Basic/Expert Dungeons & Dragons by Moldvay/Cook in 1981
- Basic/Expert/Companion/Master/Immortal Dungeons & Dragons by Mentzer in 1983
- Unearthed Arcana for AD&D (Some say that this made AD&D 1.5.)
- Advanced Dungeons & Dragons 2nd Edition in 1989
- Rules Cyclopedia for Dungeons & Dragons in 1991
A one-book compilation of Mentzer BECM D&D, some consider this the definitive version of the original Dungeons & Dragons line.
- Skills & Powers for AD&D 2nd edition in 1995
Some say that this made AD&D 2.5. It certainly is obvious, in that it changes to 12 attributes.
- Dungeons & Dragons 3.0 in 2000
- Dungeons & Dragons 3.5 in 2003
- Dungeons & Dragons 4.0 in 2008
- Pathfinder in 2009
Put out by Paizo, this version is based on the d20 SRD. It supplanted D&D 4.0 as the market leader in 2012. Some say it made D&D 3.75.
- Dungeons & Dragons 5.0 in 2014
Being a bit nitpicky... and using a "The rules are different enough that they characters have different capabilities and/or limits on advancement" meaning for edition, I'd say:
Not D&D but precursors to it:
- Chainmail with Fantasy Supplement
- Pre-6th Printing D&D (5x8's)
- 6th printing and later D&D (5x8's) - subtle changes due to renaming of Tolkein derived stuff
- either flavor of the 5x8's with the supplements I - VI - mechanically different from base box alone.
- Holmes Basic
- AD&D 1E
Note that the 1978 printing is missing some elements in the 1981 printing; each printing seems to have included some errata changes. This was caught during a discussion of the lack of clerics for elves and dwarves - they're in the 1981, but not the 1978 printing of the PHB.
- Moldvay Basic & Cook Expert
- Revised AD&D 1E (new covers and minor errata)
- AD&D 1E + Unearthed Arcana (UA makes MAJOR rules alterations to what's in the core)
- Mentzer Basic, Expert, Companion, Master, Immortal.
- AD&D 2E
- Revised AD&D 2E
- Allston D&D Cyclopedia and Denning Basic (the big black box)
- Gazeteer D&D (Mentzer, or Denning & Allston with the GAZ line or HWR line alterations)
- AD&D 2E + Player's Option series
- D&D 3.0 (d20)
- D&D 3.5 (d20)
- D&D 4.0
- D&D 4 Essentials (4.1 from what I gather)
- D&D 5e aka D&D Next.
- Pathfinder (several of the 3.X dev team jumped ship to work on it)
Putting these into families:
- Early: either little book flavor without supplements, with chainmail. VERY different feel.
- BX/BECMI/BXCMI: Moldvay/Cook, Mentzer, Denning/Allston, Gazetteer.
- 0E: little book with at least Supplements 1 & 2, or Holmes Basic
- 1E: AD&D 1E and Revised 1E,
- 1.5E: AD&D 1E+UA
- 2E: AD&D 2E and Revised 2E
- 2.5E, PO'd 2E: AD&D 2E with the player's option books.
- D20 line
- D20 D&D: D&D 3.0, D&D 3.5
- Continued under a new name: Pathfinder
- 4E: D&D 4E, D&D 4E Essentials.
- D&D Next
And that's without retroclones, pseudoclones, and knockoffs.
Note that, to me, a game edition can be any of the following:
- The core rules were significantly revised and reworded
- the characters are distinctive enough that one can readily tell which edition was in use.
- the mechanics are different enough that the character won't play the same.
Thus, the gaps between:
- Moldvay/Cook, Mentzer, and Alston/Denning, (reason 1)
- AD&D 1 and 1.5 (Reason 2 and 3)
- AD&D 2 and 2.5 (Reason 2 & 3)
- original box, and original box plus supplements 1 & 2 (Reason 2 and 3)
- Pathfinder and 3.X (Reason 2. Just look at the class skills and skill levels.)
The Acaeum details the various editions and printings. It's well worth an extended browse just for the nostalgia of the cover scans. I'm going to say nine editions: OD&D, Holmes, Moldvay, Metzner, AD&D, 2e, 3e, 3.5e, and 4e.