Research on the development of reading skills through the primary school years has pointed to the importance of individual differences in initial ability as well as the growth of those skills. Additionally, it has been theorized that reading skills develop incrementally. The present study examined the genetic and environmental influences on 2 developmental models representing these parallel ideas, generalizing the findings to explore the processes of reading development. Participants were drawn from the Florida Twin Project on Reading, with a total of 2,370 pairs of twins representative of the state of Florida. Twins' oral reading fluency scores from school progress monitoring records collected in the fall of Grades 1-5 were used to model development. Results suggested that genetic influences on the development of reading are general, shared across the early school years, as well as novel, with new genetic influences introduced at each of the first 3 years of school. The shared environment estimates suggest a pattern of general influences only, suggesting environmental effects that are moderate and stable across development.