This study examines the genetic and environmental etiology underlying the development of oral language and reading skills, and the relationship between them, over a long period of developmental time spanning middle childhood and adolescence. It focuses particularly on the differential relationship between language and two different aspects of reading: reading fluency and reading comprehension. Structural equation models were applied to language and reading data at 7, 12, and 16 years from the large-scale TEDS twin study. A series of multivariate twin models show a clear patterning of oral language with reading comprehension, as distinct from reading fluency: significant but moderate genetic overlap between oral language and reading fluency (genetic correlation rg = .46–.58 at 7, 12, and 16) contrasts with very substantial genetic overlap between oral language and reading comprehension (rg = .81–.87, at 12 and 16). This pattern is even clearer in a latent factors model, fit to the data aggregated across ages, in which a single factor representing oral language and reading comprehension is correlated with—but distinct from—a second factor representing reading fluency. A distinction between oral language and reading fluency is also apparent in different developmental trajectories: While the heritability of oral language increases over the period from 7 to 12 to 16 years (from h2 = .27 to .47 to .55), the heritability of reading fluency is high and largely stable over the same period of time (h2 = .73 to .71 to .64).