Codex: Tyranids is an expansion codex book for Warhammer 40,000 3rd Edition. This codex covers the Tyranids race detailing their history, equipment, organization, special rules and powers, painting schemes, and army lists.
From the back cover:
The Tyranids are the most rapidly evolving race in the galaxy. They are super-predators destined to hunt all others to extinction. Inside you will find:
Army Lists: The complete Tyranid army list, which allows you to choose your forces for a tabletop battle. Also included are Tyranid Hive Mind powers, symbiote bio-weapons and biomorph enhancements for Tyranid creatures.
Hobby Section: Sixteen full colour pages packed with advice and tips on collecting, modelling and playing with a Tyranid army.
Genetic Modification: New rules for designing custom hive fleets, mutants and modifying Tyranid creatures to create unique species.
Special Monsters: Two infamous Tyranid creatures are described: Old One Eye and the Red Terror!