Restimulation of wells completed in the Codell formation, a tight-gas sand, has proved successful in the Wattenberg field in northeastern Colorado. Beginning in 1997, Codell refracturing evolved into a massive program involving hundreds of wells per year. To date, more than 750 Codell wells have been restimulated, increasing reserves and resulting in finding costs less than U.S. $4.23 per barrel of oil equivalent (BOE). The full-length paper summarizes the refracture program and its evolution. The candidate selection process is examined, as well as geological and operational considerations when restimulating old wells. The use of 3D fracture simulations, run in real time during the refracture treatments, and their results are discussed.