This study examined how social-emotional and behavioral (SEB) problems and competencies contribute to changes in developmental functioning among children enrolled in Part C Early Intervention (EI), a U.S. program supporting young children with developmental delays and disabilities. The sample included 1,055 children enrolled in EI from 2011-2019 (mean age at EI entry = 17 months; 64% male; 72% marginalized racial and ethnic backgrounds). Standardized developmental assessments, drawn from administrative records, characterized developmental functioning at EI entry and exit and parents reported SEB functioning. Hierarchical regression analyses revealed that SEB problems and competencies interacted in predicting change in developmental functioning from EI entry to exit. Monitoring, identifying, and addressing SEB problems and competencies may optimize developmental outcomes for young children with developmental delays and disabilities. (author abstract)