Any person who has taught Spanish as a second language or who has interacted with a non-native speaker of Spanish can easily tell that mastering the correct use of the copulas ser and estar is very difficult in both spoken and written production. But L2 learners are not alone. The Spanish copulas also present difficulty and frustration for L2 instructors of Spanish, since most pedagogical explanations of the uses of ser and estar provided in textbooks are incomplete and inaccurate. However, the acquisition of copular constructions has not received the attention it deserves in the acquisition literature, making a special issue of Bilingualism: Language and Cognition dedicated to this topic particularly welcome. A reason for the scarcity of research in this area may be related to both the linguistic complexity of Spanish ser and estar and the inadequacy of many available theoretical treatments to explain their complementary distribution. Although constructions with ser and estar are highly frequent in the input, they are grammatically quite intricate, straddling between the levels of morphology, syntax, semantics and pragmatics. Accordingly, Bruhn de Garavito and Valenzuela correctly frame their study within the current debate on interface vulnerability in language development in general, and in adult L2 acquisition in particular (Sorace, 2004; White, in press). In a nutshell, grammatical areas which require the integration of different levels of linguistic knowledge (e.g., syntax–discourse, syntax–morphology, morphology–semantics, etc.) for processing, production, or interpretation, show developmental delays and instability in monolingual and bilingual acquisition. In the case of monolingual acquisition, instability or non-target-like behavior is temporary, but in L2 grammars, instability can persist up to very advanced levels of proficiency, eventually leading to fossilization.