Although professional development is widely used to improve the impacts of early childhood education, little is known about the conditions under which such interventions promote child outcomes. This study applies newly developed methods for quantifying intervention impact heterogeneity to understand whether educators’ collective workplace stress moderates professional development’s impacts on children’s language and literacy skills, executive functioning, and learning behaviors. Within a sample of 406 children from the National Center for Research on Early Childhood Education Professional Development Study (Mage = 4.17; 50% female; 50% Black, 32% Latinx, 11% White), professional development positively impacted child outcomes in centers where educators collectively reported high workplace stress but negatively impacted child outcomes in centers where educators collectively reported low workplace stress. (author abstract)