This monograph explores the interface between syntax and its related components through in-depth investigation of a sizable portion of the grammar of Indonesian and Javanese. It can be read on two levels. Theoretically, it proposes the minimalist interface thesis that syntax-external linguistic interfaces are endowed with domain-specific operations (insertion, deletion, and type shifting) to legitimize an otherwise non-convergent result of the syntactic derivation for phonological and semantic interpretation. Empirically, the monograph substantiates this thesis from detailed analyses of four phenomena (reduplication, active voice morphology, P-stranding under sluicing, and nominal denotation). The study not only contains a wealth of new insights into comparative syntax from the perspective of Indonesian and Javanese, but also necessitates serious reconsideration of the common view of the interfaces as merely ornamental components of natural language grammar. The monograph should appeal to syntacticians, linguists interested in linguistic interfaces and the organization of grammar, and researchers on Austronesian languages.