This article reports on the multi-institutional development and validation of an instrument that attempts to operationalize Garrison, Anderson and Archer's Community of Inquiry (Col) framework (2000). The results of the study suggest that the instrument is a valid, reliable, and efficient measure of the dimensions of social presence and cognitive presence, thereby providing additional support for the validity of the Col as a framework for constructing effective online learning environments. While factor analysis supported the idea of teaching presence as a construct, it also suggested that the construct consisted of two factors-one related to course design and organization and the other related to instructor behavior during the course. The article concludes with a discussion of potential implications of further refinement of the Col measures for researchers, designers, administrators, and instructors.