Different from traditional definitions of “interlanguage” and descriptions of interlanguage production, this study defines interlanguage in terms of its outstanding linguistic characteristics at the level of abstract lexical structure, including its three sublevels: lexical-conceptual structure, predicate-argument structure, and surface morphological realization patterns (Myers-Scotton and Jake 1995). Also different from most previous studies of interlanguage, this study views interlanguage or any interlanguage system as an outcome of a language-contact phenomenon. Adopting the Matrix Language Frame model (Myers-Scotton 1993[1997]) of bilingual intrasentential codeswitching in particular and contact linguistics in general, this study identifies the “matrix language” or the grammatical frame which structures interlanguage and reviews such a matrix language as a composite (Jake 1998; Wei 2009c). By adopting the Bilingual Lemma Activation model (Wei 2002, 2015, 2020), this study explores the nature and activity of the bilingual mental lexicon during interlanguage development. Accordingly, it proposes a particular approach to the nature and sources of second language learner errors or first language transfer in second language learning. It concludes that any interlanguage system must be driven by an incompletely acquired target language system in general and by an incompletely acquired target language abstract lexical structure in particular.