Parents often wonder whether children acquiring two languages simultaneously will attain native competences in both. In order to answert this question, we must contrast bilinguals’ language development with that of monolinguals. This requires a basic understanding of monolingual first language (L1) acquisition: It is always successful, happens at a fast rate and uniformly in that children proceed through identical developmental phases. These properties can be explained if we assume that L1 acquisition is guided by an innate Language Making Capacity (LMC), comprising general-purpose and language-specific cognitive principles. The latter make up the The Language Acquisition Device (LAD). Unversal Grammar (UG), a subcomponent of the LAD, consists of principles specifying formal properties of human languages. The simultaneous acquisition of two languages from birth exhibits the same properties as L1 development. Developmental sequences are non-distinct in bilinguals and monolinguals, and bilingual development happens at appproximately the same rate. It can thus indeed be qualified as bilingual first language acquisition.