Children's University (CU) aims to improve the aspirations, attainment, and skills of pupils aged 5-14 by providing learning activities beyond the normal school day. This trial focused on pupils in Years 5 and 6 (aged 9-11), and activities included after-school clubs, visits to universities, museums, and libraries, and 'social action' opportunities such as volunteering in the community. Local CU teams worked with schools to identify opportunities, and organise and monitor the activities. Children volunteered to take part and selected the activities they wished to attend, with the target of completing at least 30 hours of activity per year. Participation in activities was rewarded through credits, certificates, and a 'graduation' event attended by parents. 68 primary schools participated in this efficacy trial from March 2014 until July 2016. 2,603 pupils reported in an initial survey that they would like to take part in the kinds of activities offered, and these 'volunteer' pupils formed the main comparison groups. 1,452 of these pupils were in the 36 schools randomly allocated to receive the CU intervention, and 1,151 were in the 32 schools randomised to the control group. This project evaluated the impact of CU on pupils' reading and maths in Key Stage 2 tests, and on non-cognitive outcomes such as 'teamwork' and 'social responsibility' measured through an attitude survey. Results were obtained for Year 6 pupils after one year and for Year 5 pupils after two years. The headline findings below are based on the results for the 1,258 Year 5 pupils after two years. Surveys and interviews were conducted to explore other aspects of the intervention such as: participants' feedback, challenges of implementation, and control group activity. This trial was jointly funded by the Cabinet Office. Key Conclusions include: (1) Children in the CU schools made 2 additional months' progress in reading and maths compared to children in the other schools. The finding for maths has moderate security, and the finding for reading has low to moderate security; (2) Children in the CU schools made 2 additional months' progress in reading and maths compared to children in the other schools. The finding for maths has moderate security, and the finding for reading has low to moderate security; (3) Children ever eligible for Free School Meals (FSM) made 1 additional month's progress in maths, no additional progress in reading, and small gains in 'teamwork' and 'social responsibility' compared to ever FSM children in the other schools. The smaller number of ever FSM pupils means these results are less secure than the results for all pupils; (4) Compared to pupils in the control group, those in the treatment group were more likely to select professional occupations as their future aspiration, and to report higher levels of communication, empathy, self-confidence, resilience, and happiness, after the intervention, and (5) The intervention was feasible to run with support from school leaders. However, 7 schools decided not to implement the intervention despite receiving the training because of pressures to meet performance targets, and limited time. CU activities were reported as attractive to pupils. Children receiving CU for one year (those in Year 6) made a small gain in maths, but less progress in reading, compared to children in control schools. The smaller size of these effects compared to the results for the Year 5 cohort (that received CU for two years) suggests that children may benefit from longer involvement in CU. Further analyses suggested that all Year 5 children in schools doing CU made better average progress than children in control schools, regardless of their actual level of participation in CU activities. This could be seen as adding further caution to the headline results as to the extent to which gains were due to direct involvement in CU activities. However, one of the original research questions was whether the availability of CU activities could affect the whole cohort. For the non-participating group this may be evidence of such a cohort 'effect'.