Researchers have devoted substantial attention to the use of student test score data to measure teacher performance. In response to recent policy interest in using student achievement data to measure the contributions of school administrators as well, this paper investigates the capacity of longitudinal achievement data to uncover principal effects. Building on prior research, it develops multiple models for capturing the contributions of principals to student test score growth, examines the properties of each model, and compares the results of the models empirically. It then assesses the degree to which the estimates from each model are consistent with measures of principal performance that come from sources other than student test scores, such as school district evaluations. It finds that the approach that attributes the school’s effectiveness to the principal more closely aligns with non-test-based measures than do approaches that more convincingly separate the effect of the principal from the effects of other school inputs.