The Myers-Briggs Type Indicator's (MBTI) extensive use in businesses, educational institutions and in career counseling coupled with growing criticism regarding its validity demands that critical research answer the question of what it is that the MBTI measures. This research investigates three models of the underlying factors of the MBTI items (the MBTI Model, a model incorporating the five-factor model of personality (Costa & McCrae, 1991), and a model based on an exploratory factor analysis (Sipps, Alexander, & Friedt, 1985)). One thousand and thirty individuals (N men = 407; N women = 753) completed the MBTI while participating in a career development service. Confirmatory factor analysis results indicated that although the MBTI Model explains responses slightly better than the five-factor model, it is not (in its present form) optimally explanatory. These results and exploratory factor analysis results identify areas where instrument changes and additional research could lead to a more valid and reliable instrument.