Group psychotherapy has been shown to be an effective modality for the treatment of a variety of mental disorders. While the majority of the group psychotherapy research focuses on measuring the outcomes of treatment, a growing trend in group psychotherapy research focuses on describing the processes that contribute to positive or limited outcomes. This paper describes the development of the Group Process Inventory, a questionnaire instrument that structures and identifies therapeutic processes. Building upon previous research and theory, this instrument incorporates diverse data into a broad, brief, and practical instrument to measure the forty-eight prominent factors of group process. The methods section outlines psychometric decision choices used in the instruments development. The researcher drew upon empirically supported process research, theoretical hypotheses, and consultation with both experienced clinicians and researchers. The instrument was designed to be applicable across multiple theoretical group approaches including psychoeducational, cognitive-behavioral, and psychodynamic, as well as being applicable across populations including issues of diagnosis and diversity. The questionnaire was developed to be completed by group participants, although it can also be easily adapted for use by group leaders and third party raters.