The aim of this prospective study was to identify preschool factors that are associated with children's classroom engagement during their 1st school year. The study was guided by a social-motivational process model that highlights the importance of parent-child and teacher-child relationships in promoting engagement. In preschool, parents and teachers completed questionnaires assessing children's (n = 562) parent-child and teacher-child relationships, global self-concept, and mental health problems. Teachers rated children's engagement levels 1 year later in kindergarten. Research Findings: Results from structural equation modeling suggested that experiencing good-quality relationships with parents and teachers and positive self-concept during preschool were only indirectly associated with children's kindergarten classroom engagement through their negative associations with hyperactivity/inattention; only hyperactivity/inattention had a significant direct (small-moderate and negative) association with children's engagement. Practice or Policy: Interventions that improve adult-child relationships may reduce childhood hyperactivity/inattention during preschool and potentially improve children's engagement, helping them start school ready and eager to learn.