School Effectiveness (SE), as a research paradigm and, more widely, as a set of political practices in school management and development, is examined in terms of the concept of 'reductionism'. The article serves to systematise an ongoing critique of Effectiveness by Ball, Morley, Fielding, Slee, etc. Building upon studies by the biologist Steven Rose and colleagues of reductionism from psychology to biology, the reductionism of the Effectiveness discourse is analysed in its methodological, contextual, historical and moral aspects. Finally, the article explores the impact of SE upon the dominant model of School Improvement in England in particular, and the need for transformation within that paradigm.