What system do Pathfinder products use?
Currently, all Pathfinder products use the 3.5 OGL. In August 2009, after the Pathfinder RPG is finished, we’ll begin using it for Pathfinder products. Since the Pathfinder RPG is designed to be backward compatible with 3.5, players will find that all Pathfinder products are easy to use with either 3.5 or the Pathfinder RPG.