The d20 System is the ruleset behind Dungeons & Dragons 3rd edition and Dungeons & Dragons 3.5. The core mechanic of the system is rolling a d20, adding relevant bonuses or penalties to the result, and then comparing the total to a target number.

After the d20 System was released for public use via the Open Game License, it became the basis for a large number of other games, some of which were variations of Dungeons & Dragons (such as Paizo's Pathfinder) and others that were very different from the original game (such as Green Ronin's superhero game Mutants & Masterminds).

Notable third-party d20 System publishers[edit | edit source]

Publisher Major D&D compatible products
AEG Rokugan Campaign Setting (3.0)
Swashbuckling Adventures (3.0)
World's Largest Dungeon (3.5)
Atlas Games Penumbra line (3.0, 3.5)
Nyambe: African Adventures (3.0)
Eden Studios Liber Bestarius: The Book of Beasts (3.0)
Fantasy Flight Games Legends & Lairs line (3.0, 3.5)
Fast Forward Entertainment The Encyclopedia of Demons & Devils (3.0)
Goodman Games Dungeon Crawl Classics line (3.5)
Green Ronin Publishing Freeport line (3.0, 3.5)
The Book of Fiends (3.5)
The Book of the Righteous (3.5)
Kenzer & Company Kingdoms of Kalamar line (3.0, 3.5)
Malhavoc Press The Book of Eldritch Might (3.0)
Ptolus (3.5)
Iron Heroes (3.5)
Mongoose Publishing Slayer's Guide line (3.0, 3.5)
Encyclopaedia Arcane line (3.0, 3.5)
Necromancer Press The Tome of Horrors (3.0)
Paizo Pathfinder line (3.5)
Paradigm Concepts Arcanis line (3.0, 3.5)
Privateer Press Monsternomicon (3.0)
Sovereign Press Dragonlance line (3.5)
Sword & Sorcery Studios Ravenloft line (3.0, 3.5)
Creature Collection (3.0)
Scarred Lands line (3.0, 3.5)
Zeitgeist Games Dave Arneson's Blackmoor (3.5)
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