Past research demonstrates that first language (L1)-like competence in each language can be attained in simultaneous acquisition of bilingualism by mere exposure to the target languages. The question is whether this is also true for the "weaker" language (WL). The WL hypothesis claims that the WL differs fundamentally from monolingual L1 and balanced bilingual L1 and resembles second language (L2) acquisition. In this article, these claims are put to a test by analyzing "unusual" constructions in WLs, possibly indicating acquisition failure, and by reporting on analyses of the use of French by bilinguals whose dominant language is German. The available evidence does not justify the claim that WLs resemble L2. Instead, it shows that WL development can be delayed, but does not suggest acquisition failure. Finally, reduced input is unlikely to cause acquisition failure. The fundamental issue at stake is to explore the limits of the human language making capacity.Past research on child bilingualism has provided a large body of evidence that strongly supports the claim that simultaneous acquisition of two or more languages can justly be qualified as an instance of multiple "first language" (L1) acquisition (see de Houwer, 1995;and Meisel, 2004, for state-of-the-art summaries). The most crucial findings on which this assessment is based are that (a) linguistic systems are differentiated from early on, (b) grammatical development proceeds through the same developmental sequences as in monolingual acquisition, and (c) grammatical knowledge ultimately attained in each of the languages of bilingual children is identical in nature to that of their respective monolingual counterparts. It is plausible to assume that all three properties can be attributed to capacities made available by the innate human language faculty. Within the theoretical framework of universal grammar (UG), one can thus conclude that multilingual as well as monolingual acquisition is guided by principles of UG.The fact that multiple L1 development can be achieved by mere exposure to more than one language, without specific coaching or other types of additional