This book is a critical assessment of the knowledge base on educational effectiveness, covering a period of five decades of research. It formulates a "lean" theory of good schooling, and identifies and explains instances of "ineffectiveness", such as low effect sizes of malleable conditions, for which expectations are highly strung. The book presents a systemic outlook on educational effectiveness and improvement, as it starts out from an integrated multi-level model that comprises system level, school level and instructional conditions. It offers a classification of school improvement strategies and scenarios for system level educational improvement. Above all, the analysis is very systematic, comprehensive and strongly grounded in theory. The book includes a case study analysis of various strands of improvement-oriented educational policy in the Netherlands as an illustration of some of the arguments used.