Social-behavioral functioning during early childhood is associated with children's academic and social success concurrently and over time. This study explored how concurrent, year-to-year, and sustained parent--teacher and student--teacher relationships predicted children's social skills and problem behaviors across the preschool to Grade 1 transitions. Participants were 233 children (M = 5.32 years [SD = 0.27] in preschool), their parents, and their preschool (n = 65), kindergarten (n = 116), and first grade (n = 117) teachers enrolled in low-income public schools in rural and urban communities. Research Findings: Children's relationships with teachers were associated with social-behavioral functioning immediately and over time. Positive, sustained relationships from preschool through first grade predicted social-behavioral benefits. Conflictual relationships related to higher problem behaviors. Parent-teacher relationships as reported by teachers predicted children's positive social-behavioral functioning in the same year. Parents' reports of close relationships with teachers predicted more problem behaviors in the following year. Sustained relationships between parents and teachers during the transition from preschool through first grade predicted improved social skills and fewer problem behaviors over time. Practice or Policy: Providing targeted training and support for educators to develop and maintain relationships with students and parents can improve social-behavioral outcomes for children across the preschool to 1st grade transition.