This work explores the psychological theory underlying methods of intervention in cognitive development. The authors, strive to show how the practical expression of such methods can lead to long-term gains in academic achievement in ordinary schools. Within a discussion of various attempts to "teach thinking", the design, delivery and results of the "Cognitive Acceleration through Science Education" (CASE) project are described. Other programmes such as Feuerstein's "Instrumental Enrichment" are also described, to abstract the features of successful intervention programmes. Key implications are also discussed for: teaching methods; the nature of the curriculum; teacher education; and educational policy at school, local and national levels. Having established the distinction between intervention and instruction, the authors go on to show how a population, and the learning demands made upon it, can be described in terms of levels of cognitive delvelopment. The emphasis then turns to how the current profile of thinking in schools can be changed through constructivist and metacognitive strategies.