My dissertation follows the legacies of nineteenth-century scientific metrics of human development and racial difference into histories of twentieth century developmental science and medicine. In particular, I track how nineteenth-century theories of race as differential degrees of developmental plasticity, capacity, and synchronicity were reproduced and reconfigured in twentieth-century pediatric medicine and developmental psychology. I historicize the production and popularization of developmental metrics that are often perceived as race-neutral, including pediatric growth charts, developmental evaluations like the Denver Developmental Screening Test, and diagnostic criteria for developmental disabilities. More specifically, each chapter interrogates the racial parameters of developmental knowledge production and their material effects on Black parents and children. I pursue this work across multiple sites and sources, including populational growth studies, observational child studies, developmental screening tests, baby memory books, popular parenting literature, “compensatory” programs and special education classrooms in predominantly Black school districts, and pediatric well-child visits.