The Good Behaviour Game (GBG) is a universal behaviour management intervention that aims to to improve pupil behaviour with the following core elements: classroom rules, team membership, monitoring of behaviour, and positive reinforcement (rewards). While it is primarily used with children in primary schools, it can also be implemented in early years and secondary education settings. Over the course of implementation, it is intended that there is a natural progression in terms of the types of rewards given (from tangible rewards such as stickers to more abstract rewards such as free time), how long the game is played for (from 10 minutes to a whole lesson), at what frequency (from three times a week to every day), and when rewards are given (at the end of the game, the end of the day, and the end of the week). A randomised controlled trial design was used in which 77 schools were randomly allocated to implement the GBG for two years (38 schools) or continue their normal practices (39 schools). The target cohort was pupils in Year 3 (aged 7-8) in the first year of implementation (N=3084). The project was designed as an efficacy trial. Alongside the assessment of outcomes, the evaluators undertook a comprehensive mixed-methods implementation and process evaluation involving observations, interviews and focus groups. Findings showed: (1) no evidence that the GBG improves pupils' reading; (2) no evidence that the GBG improves pupils' behaviour (specifically, concentration problems, disruptive behaviour, and pro-social behaviour); (3) implementation was variable and in particular, the frequency and duration with which the GBG was played did not reach the levels expected by the developer; (4) higher levels of pupil engagement with the game were associated with improved reading, concentration, and disruptive behaviour scores at follow-up; and (5) tentative evidence that boys identified as at-risk of developing conduct problems at the beginning of the project benefitted from the GBG.