The purpose of this study was to examine the comparative influence of college administrator credential programs, on-the-job experiences, and the ISLLC Standards in the development of leadership expertise among urban public school principals. An exploratory, ex-post-facto research design used both quantitative and qualitative approaches. A survey of 101 randomly selected urban school principals from 25 of America's largest metropolitan school districts was given, followed by telephone interviews with a subset of 20 randomly selected survey participants. Data were analyzed through the use of inferential and descriptive statistics and descriptive narratives. On-the-job experiences were significantly more important in developing leadership expertise than college credential programs on each of 41 ISLLC-based learning tasks. However, college credential programs are also important sources of leadership development. Several significant differences were found between comparative ratings of learning tasks by subgroups (experience, gender, school type). Respondents ranked 78% of the ISLLC Standards as very important to the field of school leadership. Interview subjects expressed concern about lack of preparation in budgeting, data analysis, teacher evaluation, and change management. (Contains 8 tables and 3 footnotes)