OBJECTIVE: The authors aim was to examine associations of breastfeeding duration and exclusivity in infancy with executive function, behavior, and social-emotional development in mid-childhood. METHODS: The authors studied 1037 participants in Project Viva, a prebirth cohort that enrolled pregnant mothers from 1999 to 2002 and followed children for 7 to 10 years. Main exposures were: (1) duration of any breastfeeding in the first 12 months and (2) duration of exclusive breastfeeding in the first 6 months. Main outcomes were child executive function, behavior, and social-emotional development, assessed by (1) the Behavior Rating Inventory of Executive Function (BRIEF) and (2) the Strengths and Difficulties Questionnaire (SDQ), completed independently by parents and teachers. Higher scores indicate greater problems. RESULTS: In linear regression models adjusted for sociodemographics, maternal intelligence, home environment, early child care, and maternal depression, longer breastfeeding duration was not associated with substantially better executive function, behavior, or social-emotional development. For example, for each additional month of any breastfeeding, the BRIEF Global Executive Composite score (parent) was 0.10 points higher (95% confidence interval, -0.01 to 0.22) and the SDQ total difficulties score was 0.06 points higher (-0.01, 0.12). Breastfeeding duration was also not associated with BRIEF or SDQ subscales, nor was exclusive breastfeeding duration associated with any of the outcomes analyzed. CONCLUSION: Despite beneficial effects on general intelligence, longer duration of any breastfeeding or of exclusive breastfeeding was not associated with better executive function, behavior, or social-emotional development in mid-childhood.