This systematic review will focus on curriculum based social and emotional learning (SEL) interventions delivered in preschool or primary/elementary schools aimed at improving social and emotional skills among pupils. It will include any universal programme, delivered on a whole-class or school basis. The interventions' primary goal must be to improve social and emotional competence and be delivered in a pre-school/kindergarten or primary/elementary school setting as part of the normal school day. The objectives of the review are: (1) to identify, appraise and synthesise evidence from randomized and cluster randomized trials on the effectiveness of universal, school- based social and emotional learning programmes for young children age 3-11 as compared to no intervention or inactive control, for improving children's social and emotional skills and behavior; (2) to identify and classify the core components of programmes and to assess which of these components, if any, can be considered as 'active ingredients' in terms of their presence or absence being associated with differential programme effectiveness; (3) to ascertain whether fidelity of delivery and the stage of the programme's development are associated with programme effectiveness; (4) to assess if the programmes are differentially effective for different subgroups in relation to age, gender and socio-economic background; and (5) to develop a set of clear and specific recommendations, based on the evidence reviewed, to guide the further development and delivery of universal school-based social and emotional learning programmes.