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 mecahnic. 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 & Dragon in 1977-1979
- Basic/Expert Dungeons & Dragons by Moldavy/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.
Dungeons and Dragons is a game designed by Dave Arneson and Gary Gygax; there was only one edition. Gygax went on to write Advanced Dungeons and Dragons, of which there was also a single edition.
Various companies have printed game rules with the same name.
Edit: there's been some unhappiness with this answer so I'll expand it a bit. "Dungeons and Dragons" is a trade name which has been owned by various companies. As a trade name it can legally be applied to a box of cereal or a car or, more likely, a game of fantasy miniature combat or a simple "Dungeon!" style board game. In other words, it does not designate any specific game any longer.
What it originally designated was a role-playing game by Arneson and Gygax. While later applications may change, the originality of that first application is set in stone - that game, consisting of the "little brown books" will always be the original sense of the name. A similar argument can be made for the use of "Advanced Dungeons and Dragons".
So, the question is whether other uses are the same game or something else with the same name? Legally, they are different games - this was the core argument for separating Gygax from TSR-era game design royalties. Morally, neither Arneson nor Gygax voluntarily gave up their control, they were forced out at different times.
So, neither legally nor morally can other games with the same name be said to have any more connection with the original game than any retro-clone or other product with the name "Dungeons and Dragons" that I or anyone else might design. All they have is legal protection from the name being applied to other items, no matter how close to the original rules those other things may be. In that sense, which is actually quite a wide sense, those games are not, and cannot be, new editions, they are simply new product lines by the owner-of-the-moment who may apply it tomorrow to a themed tiddly-winks set.
The vast gulf between the original game and the so-called Fourth Edition demonstrates this pretty clearly - the name is meaningless as a way of determining whether something "really" is Dungeons and Dragons; it's just a marketing term.
Thus, there is only one edition of Dungeons and Dragons - the original - and one of Advanced Dungeons and Dragons - the original. Indeed, it's not clear what "fifth" edition is supposed to be the fifth edition of, since it's not actually called "Advanced Dungeons and Dragons". If the original creators had issued new rules, then they would have been genuine editions at least in the moral sense.