This article is concerned with children who grow up learning two languages simultaneously. These are children who are exposed to two languages on a regular and consistent basis beginning within the first year of birth. (Children who begin learning a second language after 1 year of age are not considered because their pattern of development may be quite different from that of simultaneous bilinguals.) Understanding the nature of and developing appropriate treatment for impairment in simultaneous bilingual acquisition (i.e., bilingual impairment) is of the utmost importance because of the large number of such children worldwide. This article discusses key features of normal bilingual acquisition and factors that can influence bilingual acquisition that do not implicate impairment. It is imperative to understand normal bilingual development if valid diagnosis and treatment of bilingual impairment is to occur. Research on impaired bilingual acquisition and its treatment are then discussed.