Children's peer-related social skills in early childhood set the stage for school readiness, social competence, and lifelong well-being. Preschoolers spend most of their classroom time in free choice, offering substantial opportunities for learning experiences that facilitate peer interactions that support social development. Yet, little is known about how early childhood teachers can best optimize children's social learning without disrupting the benefits of play during this time. The purpose of this study is to investigate how preschool teachers' social support behaviors during free choice align with children's social needs (i.e., fit of teacher-child social interaction) and the association between the fit of teacher-child social interaction and children's peer-related social behaviors. This study had two research questions: (1) To what extent is the fit of teacher-child interaction during free choice? and (2) To what extent is the fit of teacher-child social interaction during free choice associated with children's peer-related social behaviors during free choice? 30 teachers and 68 preschoolers were observed during free choice in their classrooms. Additionally, teachers were asked to complete a teacher report of child peer-related social competence. To address the research questions, descriptive statistics, and a multilevel regression model were used to investigate the association between the fit of teacher-child social interaction and child peer-related social competence during free choice, accounting for children nested within teachers. The average fit across the sample of teacher-child social interaction was of medium quality during free choice, which indicates that some teachers often provided social support that matched children's social needs during free choice, but others did not. Additionally, using the multilevel fixed effects regression model, I found that the fit of the teacher-child social interaction was significantly associated with the observed child's social behaviors. However, teacher-reported children's social behaviors were not significantly associated with the fit of teacher-child social interaction. This study provides evidence about how teacher's support aligns with children's social needs during free choice and how it associates with children's social competence. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]