Student-teacher relationships that improve over time may help slow or prevent declines in student motivation. In a diverse sample of 1,274 middle and high school students from three schools, this mixed-methods study found that those who improved in developmental relationships with teachers reported greater academic motivation, and more positive perceptions of school climate and instructional quality. Improvements in teacher-student relationships had some positive effects on students' GPAs but they varied by school as well as by aspect of the relationship measured. No differences by poverty status were seen in any of these results. Student focus groups yielded additional understanding of the actions and mechanisms through which student-teacher relationships improve. Results of this study suggest that if individual educators and entire school communities focus on strengthening student-teacher relationships, significant improvements can be made in students' motivation, engagement, and performance. [This paper has been accepted by the "Journal of Early Adolescence."]