Language acquisition rarely begins at birth for deaf individuals. The consequences of linguistic isolation in early childhood have been investigated from two perspectives. One literature documents the gestural communication systems, called homesign , that deaf children generate prior to any exposure to language. A second literature contrasts the language processing abilities of deaf adults who did or did not have access to a signed language from birth. There is now ample evidence that late learners of a signed language exhibit processing deficits relative to native signers and second language signers, hearing or deaf. This article brings together the research from these two domains to explore why use of a homesign system during early childhood does not support sign language acquisition in adulthood in the way that a first language supports second language acquisition.