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
| 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 Games | 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) |
