A review of studies on effective principals reveals that a number of methodological problems have prevented the accumulation of findings adequate to producing well developed models of instructional leadership and its effects on school outcomes. Problems of measurement and research design in the existing literature are reviewed and suggestions for future studies made. These suggestions point to future research meeting three standards: (1) measures of principals' leadership behavior must be better grounded in the behavioral processes found in schools; (2) measures of school effectiveness must be made less unidimensional and more valid and reliable; and (3) research designs connecting leadership and effectiveness should become more sensitive to issues surrounding the demonstration of causality and the potential for interactions. (Author/GK)