Critics complain that the educational leadership researchers have not produced evidence that demonstrates that principal preparation programs affect student achievement. This study addressed this complaint by focusing on the impact of one exemplary program on graduates and the schools they led. The preponderance-of-evidence strategy used in this qualitatively driven mixed-methods study uncovered apparent linkages between student achievement, principal behavior, and the principals' preparation program. One finding suggested that the program's cohort design produced lagged-socialization effects after the actual program ended. The study also demonstrated that, because of inevitable selection effects, answering impact questions is much more difficult than critics assume. (Contains 3 notes, 5 tables, and 1 figure.)