The present volume presents six papers, which study a number of topics in Basque by bilingual Basque-Spanish children growing up in the Autonomous Basque Region in Spain. The authors have dealt with a number of subjects, which range from lexical to genuinely syntactic issues, whereby all papers address the issue of whether one of the languages acquired by a child influences the other. The language combination studied in the present volume offers plenty of opportunities to inquire whether cross-linguistic influence occurs and which factors may determine or explain its duration, language internal or structural issues and language external or sociolinguistic factors. For the study of any phenomena that concern the Basque and Spanish languages, it is important to mention that these two languages have coexisted for many centuries, which has resulted in a number of contact phenomena that are already grammaticalized (see below). For the study of early language acquisition and language learning at school, it is of the utmost importance to take the complex sociolinguistic environment into account when drawing conclusions and making generalizations on the basis of a limited corpus. In what follows, we will briefly present the issue of cross-linguistic influence as it is discussed at present, we continue with a description of the most important grammatical facts and will finish with the actual sociolinguistic situation, which includes a historical and synchronic perspective.Let us begin with the issue of cross-linguistic influence. The growth in linguistic literature, which has been devoted to different acquisition types within bilingualism in the last two decades or so, has led us to rethink the issue of research in the field of language acquisition in a number of ways, that is, themes to be studied, methodological issues and ultimately interpretation of primary data. A variety of questions has been discussed in the past, such as whether bilinguals attain the same level of competence in both languages and whether bilingual acquisition is more costly than monolingual acquisition. One of the most controversially discussed papers in bilingual language acquisition has been the seminal work by Volterra and Taeschner (1978) who proposed that bilinguals, initially, had one lexicon for both languages, which splits into two at a second stage. The second stage is also characterized by the existence of one grammar which splits into two at a later stage. The gradual emergence of two languages out of one lexicon as suggested by Volterra and Taeschner has been severely criticized due to the methodological and conceptual shortcomings of the article over many years and was abandoned in terms of explanatory adequacy. However, any scholar working in the area of bilingual acquisition was aware of the fact that, although ultimate attainment in monolingual and bilingual acquisition was in most cases identical, some stages between onset and ultimate attainment in bilingual settings contained features that could be attributable to one of the two languages being acquired and, hence, required some attention rather a mere mention. However, the divergences observed between monolingual and bilingual acquisition had not been a matter of any theoretical discussion until Hulk and Muller (2000) and Muller and Hulk (2001) proposed that the languages acquired by bilinguals could influence each other provided some language internal constraints were met. Cross-linguistic influence is expected to occur:1. In the syntax-pragmatics domain if2. the languages at stake differ in structural terms but share some superficial properties.Based on these premises, a great deal of research in various language combinations (Basque- Spanish, German-French, Spanish-English, German-Italian and German-English bilingualism to quote the most well-known combinations) has been conducted, which was able to show that crosslinguistic influence in the alluded domain exists and, more importantly, is attributable to the other language being acquired (Kupisch, 2008; Larranaga & Guijarro-Fuentes, 2012; Muller & Hulk, 2001, to cite but a few). …