Little is known about the underlying causes and developmental patterns of stability and change in early reading abilities. In a longitudinal study of twins (n=4,291 pairs), individual differences in reading achievement assessed by teachers using U.K. National Curriculum (NC) criteria showed substantial heritabilities at ages 7, 9, and 10 years (0.57-0.67) and modest shared environmental influences (0.10-0.17). Stability in NC scores was primarily mediated genetically. There was also evidence for age-specific genetic influences at each age. Genetic influences on reading are substantial and stable during the elementary school years despite the shift from "learning to read" to "reading to learn."