This article describes the development and initial reliability and validity estimates of the Gambling Treatment Outcome Monitoring System (GAMTOMS), a multi-instrument and multidimensional outcome assessment battery designed to measure gambling treatment outcomes. Reliability methods included both internal consistency of scales and a 1-week test–retest temporal stability procedure. Validity was examined with procedures to estimate content, convergent, discriminant, predictive, and construct validity. Data were collected from 2 separate studies, 1 on the questionnaire version and 1 on the interview version. The questionnaire study included 46 female and 41 male gambling treatment clients and 22 female and 2 male nonclinical participants. The interview study included 88 female and 62 male gambling treatment clients and 16 female and 9 male nonclinical participants. The GAMTOMS was found to yield satisfactory estimates of internal consistency, and 1-week test–retest reliability and was also found to demonstrate satisfactory content, convergent, discriminant, predictive, and construct validity.