This paper reports on a study of the effect of daylighting on human performance. In this project, we established a statistically compelling connection between daylighting and student performance. The methodology used to conduct this research is described and the results are presented and interpreted. For the schools study, data sets of student performance on standardized tests were correlated with information about the demographic characteristics of the students, the operation of the schools, and the physical conditions of classrooms. Each classroom was assigned a series of codes indicating the size and tint of its windows, the presence and type of any skylighting, and the overall amount and uniformity of daylight expected. The analysis included data on over 21,000 students in three distinctly different school districts. The three districts have different curricula and teaching styles, different school building designs and very different climates. A supplemental follow-up study explored the question of teacher bias in classroom selection The findings of this study indicate better student test scores in daylit classrooms, and suggest that there is an important relationship between daylight availability in buildings and human performance. Possible causal mechanisms are discussed, along with limitations of current findings and future research directions. The paper also presents an important methodology that may be applicable to other areas of lighting research.