The present study addresses the distinction between specific (SLI) and nonspecific (NLI) language impairment at an etiological level by estimating the relative genetic and environmental contributions to language impairment in children with SLI and NLI. Drawing on a large longitudinal twin study, we tested a sample of 356 four-and-a-half-year-old children with low language ability and their twin partners at home on a range of language and nonverbal measures. For children whose language and nonverbal abilities were both low (NLI), genetic influence on language impairment was moderate and shared environmental influence was substantial. A similar pattern emerged for children whose language difficulties occurred in apparent isolation (SLI), although there was a trend for the genetic effects to be smaller for SLI than for NLI: Group heritability was .18 for SLI and .52 for NLI. Probandwise cross-concordances were suggestive of some genetic overlap between these two groups, but not with a subgroup of children with more severe cognitive delay.