This is a report of a pilot study of the Physically Active Lessons (PAL) programme, which was piloted in five primary schools in Hertfordshire between February and July 2014. PAL involves adapting lesson plans to combine short bursts of physical activity with academic content. The study had two aims: (1) to explore the feasibility of adapting a programme of physical activity and implementing it in the classroom setting, and (2) to explore the feasibility of a future, large-scale trial to provide a proper, robust test of the impact of the programme in improving attainment. Within each of six primary schools, Years 4 and 5 were randomly allocated to either intervention or control groups (472 pupils in total). This was a post-test only design and data was collected in July and October 2014. A process evaluation was conducted alongside the trial in order to provide data on programme implementation and to capture the perceptions and experiences of key stakeholders' engagement with the programme. Key areas explored were the quality of implementation of the intervention, fidelity, and perceived programme outcomes. All headteachers, as well as intervention and control teachers in participating schools, were invited to take part. Three focus groups were conducted with 6-10 children in each. Key conclusions include: (1) Evidence for the feasibility of implementing Physically Active Lessons in the classroom is mixed; both teachers and children reported enjoying the programme, however, the programme created a lot of additional work for teachers and thus, enthusiasm tended to fade towards the end of the study; (2) Teachers found the training enjoyable and helpful, however, they reported that it was sometimes difficult and often time consuming to develop lesson plans and think of ways to integrate physical activity into lessons, especially literacy lessons; (3) More work is needed to develop the programme before it is ready to be tested using a full trial. Additional work is required to develop a programme manual and integrate additional support mechanisms for teachers throughout the delivery period of the intervention; and (4) If the programme is taken to a full effectiveness trial it is recommended that paper-based tests are used, additional explanatory variables are measured, and randomisation can successfully occur at the year-group level. [The project was co-funded by Nike, Inc. as part of the "Designed to Move" initiative.]