Many courses aim to promote reflective thinking or reflection upon practice, but there is a scarcity of readily usable instruments to determine whether students engage in reflective thinking and, if so, to what extent. This paper reports the development and testing of such an instrument. To ensure validity, the constructs measured were derived from the extensive literature on reflective thinking, particularly the writing of Mezirow. A combination of the literature review and initial testing led to the development of a four-scale instrument measuring four constructs; habitual action, understanding, reflection and critical reflection. The final version of the instrument was tested with a sample of 303 students from eight classes of a health science faculty. The reliability of the scales was established by acceptable Cronbach alpha values. Confirmatory factor analysis showed a good fit to the proposed four-factor structure. Comparison of mean scores between the eight classes showed predicted significant differences on each of the four scales between undergraduate and postgraduate students.