A classroom randomized trial (n = 31 classrooms) was conducted using the Second Step Early Learning (SSEL) curriculum compared to usual curricula. Head Start and community preschool classrooms enrolling low income children were randomly assigned to deliver SSEL (n = 16) or usual curricula (n = 15). Data are reported for four year olds independently assessed for executive functioning (EF) and social-emotional skills (SE) in fall and spring of the preschool year. Analyses used three level Hierarchical Linear Modeling, including two EF tasks or two SE tasks as level 1, child as level 2, and classroom as level 3. Controlling for baseline EF, SE, cognitive ability, parent income, child sex, age, and ethnicity, children receiving the SSEL curriculum had significantly better end of preschool EF skills and marginally significantly better end of preschool SE skills. The curriculum is thus promising in its potential to improve at-risk preschool children's EF and SE. (author abstract)