Good time is granted to reward prisoners who conform to prison rules, who participate in prison programs and industries, and who give blood, serve as medical subjects, or perform outstanding services. States define and administer good time credits differently. States with determinate sentencing schemes (ones without parole) use good time to accelerate the date of release. Almost all indeterminate States subtract good time from the maximum sentence. They commonly allow 1 day of good time for every 3 days of good behavior, thereby reducing a prison sentence by one fourth. Recent sentencing reforms have not changed good time laws significantly; a table shows changes in such laws for eight determinate States. Advocates justify good time as a means of maintaining order and discipline, mitigating sentence severity, and serving as a safety valve for overcrowding. Prison officials argue that good time allows an accurate prediction of the release date and encourages rehabilitation. These justifications are unsound; prior behavior is not a good predictor of future recidivism and should not be rewarded without great care. Other sentencing alternatives, such as shorter terms, should be considered instead of good time. Footnotes and graphs are also included.