The study investigated the psychometric properties of the National Association of Secondary School Principals (NASSP) 21st Century School Administrator Skills instrument. The researcher used confirmatory factor analysis (CFA) to verify the number of underlying dimensions of the instrument and the pattern of item-factor relationships, estimate the scale reliability, and examine the convergent and discriminant validity across two different groups of school leaders--principals and assistant principals. The researcher drew the sample from an existing leadership assessment data set collected for a competitive school leadership program (SLP) grant funded by the U.S. Department of Education in a mid-sized urban school district in the Northeastern region of the United States. In total, 339 teachers' ratings on their principals and 344 teachers' ratings on their assistant principals were available for analysis. The researcher performed CFA on the 67 items of the 21st Century School Administrator Skills instrument for the principal and assistant principal ratings respectively in Mplus using MLM estimation method. CFA revealed adequate goodness of fit of the hypothesized 10-factor model and high subscale reliability and convergent validity. However, the instrument also manifested severe lack of discriminant validity. A follow-up exploratory factor analysis revealed that the current instrument measures two leadership dimensions instead of ten as originally hypothesized by NASSP. The researcher suggested that the revision of the NASSP 21st Century School Administrator Skills instrument could benefit from a re-conceptualization of school leadership, a comprehensive review of extant empirical literature and principal leadership assessment tools, evaluation of existing items as well as addition of new items based on theoretical and empirical literature. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]