This 16-month project focuses on exploring the use of Kegan's adult developmental theory to explain the important differences teachers experience in the same mathematics professional development. The only current measure of this developmental difference, the Subject-Object Interview, is expensive and labor-intensive to administer. This project will develop a written instrument and methods that would provide a more economic and less labor-intensive measure of adult developmental theory and would allow for future study of the impact of developmental differences on teachers' experiences within and across professional development programs.