Loevinger's (1976) theory of ego development represents an important and original approach to understanding personality development. More than 20 years have elapsed since the last substantial critical review of the validity of the theory and its measurement (Loevinger, 1979). Our article provides an updated critical review of the theory by reviewing the considerable body of research that has accumulated over the past 20 years. This review addresses the construct, predictive, and discriminant validity of ego development theory and the Washington University Sentence Completion Test (WUSCT; Loevinger & Wessler, 1970; Loevinger, Wessler, & Redmore, 1970) as its measurement. We conclude that there is substantial empirical support for the conceptual soundness of ego development theory and the WUSCT.