Background: Recent interest in the teaching of thinking skills within education has led to an increase in thinking skills packages available to schools. However many of these are not based on scientific evaluation (DfEE, 1999). This paper endeavours to examine the effectiveness of one approach, that of infusion, to teaching thinking. Aims: To investigate the impact of an infusion methodology, activating children's thinking skills (ACTS), on the cognitive, social, and emotional development of children in Year 4-6 in primary schools. This is a sister project to research being conducted in Northern Ireland (McGuinness, 2006). Sample: The study involved 404 children from 8 primary schools in one local authority. These were divided into 160 in the experimental group and 244 in the waiting list control group. Methods: A quasi-experimental design was used with pre-, post-, and delayed post-tests to ascertain changes in children's cognitive abilities, self-perceptions, and social/behavioural skills using quantitative measures. In addition qualitative techniques were used with pupils and teachers to evaluate effectiveness. Results: The experimental group made significantly greater gains in cognitive ability skills over a 2 year period compared to the waiting list control. Qualitative data demonstrated a positive impact on children's social and emotional development. In addition teacher professional development was reported to be enhanced. Conclusions: This research indicated that children's cognitive abilities can be developed following a 2 year period of the ACTS infusion intervention. While some positive effects were evidenced on the social and emotional development of children, further study will be necessary to examine these in more detail.