Do principals make a significant contribution to the improvement of schools? Based on the results of research reported over the past 15 years, the answer is that ‘some do but many do not*. This research, recently reviewed by Leith­ wood et al. (1990), has begun to generate the information required to explain the substantial variation in principals* impact on schools. Our framework for organizing this information suggests that a broad array of variables in the principals’ environment influences how they think and feel about their job - their internal mental processes. These thoughts and feelings, in turn, give rise to their administrative practices; such practices exercise both direct and indi­ rect effects on a variety of student and other school-related outcomes.