Preschool PATHS (Promoting Alternative Thinking Strategies) is an evidence-based universal prevention program focused on promoting children's social and emotional competencies and reducing the likelihood of behaviour problems and negative relationships with peers and teachers. This paper examines changes in the social and emotional competencies of the first children to participate in Preschool PATHS in Croatia. This study included 164 children, ages 3-6, in 12 preschool classrooms in three cities across Croatia, who participated in the classroom-based Preschool PATHS curriculum. At the beginning and end of the preschool year, teachers completed well-validated and reliable assessments of social and emotional competencies on each child. Hierarchical linear models revealed statistically significant and substantial improvements in prosocial behaviour, emotion regulation, emotion symptoms, peer problems, relational aggression, conduct problems, and hyperactive-impulsive behaviour. Study findings reveal significant changes in children's social and emotional competencies during preschool. This time may present a unique opportunity to buttress children's skills and improve long-term school success through the implementation of a rigorous empirically-validated prevention program such as Preschool PATHS.