This article describes the theoretical underpiunings, construction, and validation process of an instrument designed to measure developmental processes in groups. A total of 164 employees of three health care facilities comprised the validation sample. Reliability and intemal consistency were assessed. The process of establishing content, concurrent, criterion-related, and construct validity is also reported. The results support the reliability and validity of the Group Development Questionnaire. The existence of a dependable measure of group development may stimulate more basic research of group development and may encourage the creation of more research tools in this important area of study.