Old-School Essentials is a modular game where the core rules of the game are extended by additional "rules modules" which add content for different genres of fantastic adventure. The Old-School Essentials Rules Tome is a compilation of the core rules plus four rules modules, forming a complete game for Classic Fantasy adventures.

Core Rules: Rules for character creation and advancement, adventuring in dungeons, the wilderness, and at sea, magic and combat.

Classic Fantasy: Genre Rules: Seven classic classes (cleric, dwarf, elf, fighter, halfling, magic-user, thief), complete lists of weapons and adventuring gear, extensive lists of vehicles, mounts, and vessels, mercenaries and specialists for hire, rules for stronghold construction.

Classic Fantasy: Cleric and Magic-User Spells: The complete set of 34 cleric spells (from 1st to 5th level) and 72 magic-user spells (from 1st to 6th level), for use by players or cleric, elf, and magic-user characters.

Classic Fantasy: Monsters: A selection of over 200 classic monsters to challenge adventurers of all levels.

Classic Fantasy: Treasures: A hoard of over 150 wondrous magic items.

The material in this book forms a complete clone of the classic Basic/Expert game, including all rules, classes, spells, monsters, and magic items.