This article offers the author's commentary on the Multiple Grammars (MG) language acquisition theory proposed by Luiz Amaral and Tom Roeper in the present issue. Multiple Grammars advances the claim that optionality is a constitutive characteristic of any one grammar, with interlanguage grammars being perhaps the clearest examples of a system that subsumes several different and often conflicting subgrammars made up of properties of the first language (L1), the target language and idiosyncratic rules. Topics discussed include advances in psycholinguistic research on the multilingual lexicon, the concept of second language acquisition, and the concept of optionality. It concludes by stating that the innovation and the merit in the Multiple Grammars approach lie in advancing a coherent account of the grammatical representations involved in morphosyntactic optionality. At the same time, the challenge for the model is to embed Multiple Grammars in an overall theory of L2 acquisition that constrains the use of multiple grammars.