This user guide describes the basic functionality of Ludii -- a "general game system" that can be used to play a wide variety of games.Ludii is a program developed for the ERC-funded Digital Ludeme Project, in which mathematical and computational approaches are used to study how games were played, and spread, throughout history.Primary features of Ludii include:- A large library of implemented games, which can be played by any combination of human and computer players, and displayed graphically in the application.- A new ludeme-based game description language, which allows for new games to be modelled in more succinct and human-friendly formats than prior game description languages, and run more efficiently.- Implementations of common game-...