A case study approach was adopted to investigate two thinking skills programmes for a maximum variation sample of five- to six-year-olds in four schools, in two local authorities (LAs), in England and Wales, using multiple methods. School staff interviewed felt that thinking skills programmes enhanced critical thinking skills and improved use of language, attentive listening, social cooperation, confidence and independence. Welsh staff indicated that the programmes had a whole-school impact, reflecting national policy, intensive training, coaching and monitoring and leading to transfer to other lessons, such as mathematics. Without a national curriculum mandate, the English thinking-skills practice was vulnerable to disruption and lack of continuity. Observation showed that teachers' delivery varied in open questioning and challenge, group work and pupil talk. Pupils' verbal reasoning and numerical attainments showed modest evidence of change and development. Conditions supporting change and sustainability included: national policy with thinking skills at its core; high-quality professional development and a whole-school approach; coaching with systematic observation and feedback. This promoted changes in leadership, trust and collaboration, structures of interaction between policy and LA, head teacher, teachers and children. (Contains 4 tables.)