From back of book:
"Up From the Ruins
The Final Wars destroyed civilization. Competing forces battle for control of the shattered remains, using everything from strong fists and cold steel to the most advanced science of the old world and mysterious new powers of the mind.
The Gamma World Player's Handbook presents a new edition of the classic sci-fi adventure game, energized for the 21st century! Play humans, mutants and new races created in the chaos of the Final Wars, out not just to survive but to thrive in the midst of adventure, danger, and mystery.
Requires the use of the d20 Modern Roleplaying Game or Dungeons and Dragons Player's Handbook, Version 3.5, published by Wizards of the Coast, Inc.
The Gamma World campaign setting is published under license with Wizards of the Coast and is 100% compatible with d20 rules."