The book provides an extensive look at tools to assess language competence in child and adult multilingualism, as well as at the acquisition of particular grammatical phenomena, paying special attention to the Romance languages (Spanish, French, Italian, Catalan) as majority or heritage languages. The book explores numerous methodological possibilities to determine multilinguals' proficiency in their different L1s, such as receptive vocabulary measures, spontaneous language production and (parental) background questionnaires. The book also looks at multilinguals' development of Romance (non-)mainstream languages by examining a wide range of language assessment methods and the language-external factors that might contribute to a multilingual’s linguistic competence. It provides new insights on the acquisition of Romance languages in several contexts: (i) as heritage languages in non-Romance speaking countries in Europe; (ii) as majority languages in monolingual and bilingual regions; and (iii) as school languages in bilingual education programs in Germany. The book contributes to a better understanding of multilinguals' language development, proficiency and dominance, starting in preschool, and particularly for language combinations with (at least) one Romance language. This book is intended for linguists with a special interest in Romance languages, school educators, teachers, and researchers in the field of heritage and majority languages.

Laia Arnaus Gil is a post-doctoral fellow in Romance Linguistics at the University of Wuppertal, Germany. She wrote her PhD on the early acquisition of Romance and German copula and auxiliary verbs. She participated in a research project on code-switching in early bilingualism and co-directed the research project on early trilingualism. She is currently leading a research project on Catalan, Spanish and German as heritage (HL) and majority (ML) languages. She is also currently working on her teaching degree ('Habilitation') on the (early) multilingual acquisition of Romance attributive adjectives and the language-external factors promoting HL-competence. She is member of several research groups which examine linguistic variation in multilingual grammars, family language policies and the role of different factors for HL-competence.

Amelia Jiménez Gaspar is a research assistant at the University of the Balearic Islands, Spain, where she received her PhD in Linguistics in 2021. During 2015 and 2016, she worked as a research assistant for a research project on early trilingualism at the University of Wuppertal, directed by Natascha Müller and Laia Arnaus Gil and funded by the German Research Foundation. In 2017, she obtained a grant from La Caixa Bank to enjoy a research stay at the University of Michigan (USA) with Acrisio Pires. She is currently a scientific member of the research project "Bilingual and Trilingual First Language Acquisition (BiTriFLA)" focused on Catalan, Spanish and German as heritage languages in Germany and in the Catalan countries, directed by Laia Arnaus Gil. Her research activity is focused on language acquisition and language development, taking into account the fields of sociolinguistics, bilingualism and trilingualism.