It is increasingly popular to "teach" thinking skills in schools. A diverse variety of programmes exist to support practitioners in this task, and some research has been gathered on the effectiveness of individual approaches. However, the difficulties when assessing the development of thinking skills are widely documented. This study aimed to investigate the effectiveness of teaching thinking skills explicitly to 11/12-year olds by infusing thinking skills into the curriculum (i.e., teaching thinking skills simultaneously with subject content). There were three intervention conditions: collaborative, individual and control. The effectiveness of the intervention was evaluated with a combination of standardised and study-specific pre- and post-tests. Results demonstrated statistically significant gains for both the individual and collaborative learning conditions in a range of thinking skills. The greatest increase in performance was seen in the collaborative learning condition. Educational implications for policy and practice are discussed. (Contains 2 figures and 5 tables.)