We use unique data collected in Ethiopia and Peru as part of the Young Lives Study to investigate the relationship between early undernutrition and four foundational cognitive skills, the first two of which measure executive functioning: working memory, inhibitory control, long-term memory, and implicit learning. We exploit the rich longitudinal data available to control for potential confounders at the household level and for time-invariant community characteristics. We also exploit the availability of data for paired-siblings to obtain household fixed-effects estimates. Overall, we find robust evidence that stunting is negatively related with the development of executive functions, predicting reductions in working memory and inhibitory control by 12.6% and 5.8% of a standard deviation. Our results shed light on the mechanisms that explain the relationship between early nutrition and school achievement tests suggesting that good nutrition is an important determinant of children’s learning capacities.