This highly detailed study maps four decades of evolution of the concept of what constitutes effective school leadership. It analyses the theoretical background to these developments and advocates the utility of thinking of a ?lean? form of school leadership that is comparable to the concept of ?meta-control?. A wide-ranging survey of the empirical research literature on leadership effects includes the presentation of results from earlier meta-analyses as well as a new meta-analysis on some 25 studies carried out between 2005 and 2010.