Generative grammar addressed for the first time acquisition as a central issue in the study of grammar. This perspective has given rise over the years to a considerable body of work, mainly on first language acquisition, but also on second language acquistion, bilingual acquisition, and the acquisition by children affected by SLI. If we assume continuity, i.e. that all stages in acquisition reflect possible natural languages, we must posit a mutual dependency between grammatical theory and the facts of acquisition. On the one hand, theories about particular languages should pass the proof against data from acquisition. On the other hand, acquisition data may provide a source of evidence for deciding among alternative theories of language.
Within the past twenty years, Romance languages have been an increasing focus of research for theories of language, and the literature on the acquisition of Romance has grown steadily during this time. The moment thus seemed ripe to unify the efforts of several scholars working on the acquisition of Romance languages and devote a special issue of CatJL to the acquisition of Romance, taking into consideration the double perspective just mentioned. We thank the authors and reviewers for making this project possible. The volume deals with the acquisition of the phonology, morphology and syntax in Catalan, French, Portuguese, Romanian and Spanish. The papers dealing with phonology focus on two main topics: the acquisition of prosodic constituents and the interface between phonology and morphology. The prosodic constituents examined are: the syllable (Demuth & Kehoe on consonant clusters in French; Prieto & Bosch Baliarda on codas in Catalan) and the foot (Goad & Buckley on the relevance of the foot in Québec French). The morpho-phonological interface is treated by Fikkert & Freitas (on the relevance of allophony and allomorphy for the acquisition of the Portuguese vowel system), and Lleó (on the dependence of plural marking on phonological acquisition). The papers dealing with syntax focus on the acquisition of pronominal clitics (Babyonyshev & Marin on clitic production in Romanian and Pirvulescu on clitic omission in French) and verb and wh- movement (Costa & Loureiro on verb raising in Portuguese, Hamann on wh- movement in French).
All the papers in this volume show the relevance of formal linguistics to issues in acquisition, focusing on the relationship between representation and acquisition from different perspectives and in different modules of the grammar.

While most syntactic research has naturally focused on varieties spoken by (supposedly) monolingual speakers, the study of language contact stands out as particularly relevant not only as celebration of linguistic diversity, but also from the theoretical point of view. For one thing, it expands the number of syntactic context available to test hypotheses by adding contact varieties and heritage languages as well as phenomena such as code-switching into the picture. Moreover, the study of language contact presents data collection challenges, which speak directly to ongoing debates on data collection standards in the field of syntax. In particular, there is an ongoing trend towards the adoption of experimental data collection methods and statistical analysis and/or the use of crowdsourcing and citizen science (e.g., see Gallego and Ortega-Santos 2019). Language-contact phenomena correspond to highly specific geolects/sociolects that are not necessarily spoken by the researchers. Thus, researchers are faced with the question of how to gather the data most efficiently and with generalizability; crucially, these issues are discussed explicitly in the corresponding publications, in contrast to most non-experimentalist or non-variationist syntactic research, thus helping the field of theoretical syntax as a whole adopt experimental data collection protocols. Furthermore, the study of language contact has figured prominently in the study of language change, with an emphasis on the extent to which language contact may cause syntactic changes – changes in the I-grammar, say, parameter resetting – and, if so, under what conditions (see Meisel et al. 2013 and references therein). Last but not least, this is also an area relevant to the study of external factors (e.g., language identity or language shift). As such, it lends itself to interdisciplinary work and hopefully crosspollination inthe study of the relationship between the grammar/competence and more usage-based notions such as frequency.