By combining theoretical analysis and empirical investigation, this monograph investigates the status of interfaces in Minimalist linguistic theory, second language acquisition and native language attrition. Two major questions are currently under debate: (1) what exactly makes a linguistic phenomenon an ‘interface phenomenon’, and (2) what is the specific role that the interfaces play in explaining language loss and persistent problems in second language acquisition? Answers to these questions are provided by a theoretical examination of the role that economy and computational efficiency play in recent Minimalist models of the language faculty, as well as by evidence obtained in two empirical studies examining the acquisition and attrition of two interface phenomena: Spanish subject realization and word order variation. The result is a new definition of ‘interface phenomena’ which deemphasizes syntactic complexity and focuses on the effect of interface interpretive conditions on syntactic structure. This work also shows that representational deficits cannot be ruled out in the acquisition and attrition of interface structures.