Tweet started playing RPGs in the 1970s, and in 1987, he founded Lion Rampant with his friend Mark Rein·Hagen to publish RPG material. Their first major product was Ars Magica, a game of ambitious and eccentric wizards and their long-suffering employees in medieval Europe. Ars Magica was noted for its innovative emphasis on "troupe-style" play, with players able to switch characters depending on who was active in the current scene, and for its flexible, improvisational magic system. In 1991, Lion Rampant merged with White Wolf magazine to form White Wolf, Inc., and while Rein·Hagen began developing the Old World of Darkness (which shows distinct influences from Ars Magica), Tweet developed Over the Edge for Atlas Games with Robin Laws for publication in 1992. OtE is a game of conspiracies and surreal mysteries, also noted for its stripped-down but functional game system.
Tweet also did some work on the Talislanta game line for Wizards of the Coast, and when Wizards found themselves flush with cash after the early success of Magic: The Gathering, they decided to get back into RPGs in a big way; they acquired the rights to Ars Magica, and in 1994 they hired Tweet as a game developer. His first creation for them was Everway; the innovations in this case were an emphasis on lush visuals and the use of cards in both character creation in play — all very appropriate from the company behind Magic — and a significant narrative component.
However, Wizards rather quickly pivoted away from the idea of new or radically innovative RPGs, selling both Ars Magica and Everway to other companies. They retained Tweet's services, however, and when they acquired TSR, they gave him a new task; to recreate Dungeons & Dragons, the 800 lb. gorilla of the hobby.
Tweet's creation (in collaboration with others) of Dungeons & Dragons 3rd Edition (published in 2000) stripped the game back to a set of core mechanics, discarding much of the detritus and arbitrary complexity that had accumulated around the game, and launched something of an RPG revival. At the heart of the new edition was the d20 System, a set of rule mechanics that were thrown open to anyone to employ in games, making Tweet's design the most widespread RPG rules of the early years of the century by a very large margin. However, he moved on to Wizards' miniatures division while others developed further editions of D&D, and in 2008, the company laid him off.
In 2013, Pelgrane Press published 13th Age, a game by Tweet and Rob Heinsoo that combines a large D&D influence with a more narrative-oriented, improvisational approach. Tweet has also made contributions to other game lines, and has worked on projects intended to help in teaching about Evolution: Grandmother Fish, (2015) a Kickstarter-funded book for preschoolers, and two card games, Clades and Clades Prehistoric (2018).
