The Gambling Timeline Followback (G-TLFB), a measure of gambling behavior that uses the timeline followback methodology, was psychometrically evaluated with samples of frequent-gambling young adults. Seven dimensions of gambling behavior were assessed: type, frequency, duration, intent, risk, win-loss, and consumption of alcohol while gambling. The G-TLFB demonstrated adequate to excellent test-retest reliability with a sample of 57 frequent gamblers (r=.75 to .96) and with a group of 34 disordered gamblers (r=.73 to .93). With a separate sample of 58 frequent-gambling participants, convergent, concurrent, and discriminant validity were assessed. The G-TLFB correlated positively with daily self-monitoring reports (r=.59 to .87). The dimensions of frequency and duration demonstraied concurrent validity with gambling screening instruments, and standard drinks consumed while gambling demonstrated concurrent validity with a measure of alcohol misuse. The G-TLFB also demonstrated discriminant validity with demographic variables and a measure of positive impression management.