Research Findings: Despite the abundance of research suggesting that preschool classroom quality influences children's social-emotional development, the equally important and related question of how characteristics of children enrolled in a classroom influence classroom quality has rarely been addressed. The current article focuses on this question while also considering teacher stress as a mediator of the relationship between child behavior problems and classroom emotional climate. Data came from 2 low-income samples. Ordinary least squares regression revealed that higher levels of child externalizing behavior problems in the fall predicted higher teacher stress in the spring. Teacher stress was nonlinearly related to classroom emotional climate in the spring: Moderate levels of teacher stress were associated with higher (i.e., more positive) classroom emotional climates, and low and high levels of teacher stress were associated with lower classroom emotional climates. Contrary to expectations, higher levels of child externalizing behavior problems were related to higher classroom emotional climates. There was no evidence that teacher stress mediated this relationship. Practice or Policy: These results are discussed in terms of strategies to reduce the disruptive influence of child behavior problems on the classroom emotional climate as well as strategies to limit high levels of preschool teacher stress.