The "Irie Classrooms Toolbox": a cluster randomised trial of a universal violence prevention programme in Jamaican preschools. Violence is a leading worldwide public health problem with a very high prevalence in Jamaica. Interventions in early childhood are an important component in the primary prevention of violence. Training young children's caregivers in behavioural strategies to reduce child aggression and promote child social skills can 1) prevent the early development of antisocial behaviour in children and 2) reduce violence against children by caregivers. Corporal punishment is widely used in schools across Jamaica despite efforts by the Ministry of Education to eliminate it. Although banned by law in early childhood institutions, corporal punishment is still common in Jamaican preschools. The prevalence of conduct problems in young Jamaican children has not been accurately determined but estimates suggest it is between 12%-21%. Preschool teachers report child behaviour problems as a major concern and feel ill equipped to deal with them. We have shown that teachers trained to use evidence-based child behaviour management strategies provide a more emotionally supportive classroom environment and use less corporal punishment. This translates to improvements in class-wide child behaviour; reductions in high-risk children's conduct problems and increases in their social skills at school and at home. Through several years working with preschool teachers in Jamaica, we have developed an intervention that involves guiding teachers in the use of  a "toolbox" of key behaviour management strategies that are relevant, easy to use, effective and flexible. The intervention is delivered through participatory and practical methods and has been specifically designed to be scalable, low cost and suitable for use in low resource settings with teachers with limited formal teacher training.  We now plan to evaluate this intervention in 76 preschools (228 classrooms) to determine if benefits are obtained when the training is disseminated on a large scale. The preschools will be randomly assigned to a group in which all teachers receive the intervention in year 1 (38 preschools, 114 classrooms) and a second group that will receive the intervention a year later (38 preschools, 114 classrooms). The intervention will involve 5 full-day workshops, monthly in-class support and weekly text messages over the course of a school year. We will evaluate the effect of the intervention on class-wide measures of child aggression and teachers' use of corporal punishment and verbal abuse. We will also evaluate whether the intervention benefits the quality of the classroom environment, class-wide measures of children's prosocial behaviour, teachers' mental health and children's self-regulation, mental health and attendance. We will examine issues surrounding implementation of the intervention including measuring the degree to which teachers adopt the strategies in the classroom, factors affecting teachers' uptake of the intervention and teachers' views on the components of the toolbox and the training. This will help us to refine the toolbox to ensure its' suitability for implementation at scale. An economic evaluation will be conducted to ascertain the costs associated with delivering the intervention to teachers and the ratio of costs to the benefits obtained. This study has the potential to make an enormous public health impact in Jamaica with benefits to reduced aggression and improved child mental health at the population level. We have close links with the government agencies and training institutions responsible for early childhood education in Jamaica and are confident that, if this study shows positive results, the intervention will be integrated into on-going teacher training initiatives. Furthermore, the intervention would be suitable for use at the preschool and early primary school level in other low and middle-income countries with potential to be an important component of global initiatives to prevent violence against children and to promote child mental health. The Global Challenges Research Fund (GCRF) supports cutting-edge research to address challenges faced by developing countries. The fund addresses the UN sustainable development goals. It aims to maximise the impact of research and innovation to improve lives and opportunity in the developing world.