Multilingualism matters in a world characterised by widespread linguistic and cultural diversity. Indeed, multilingualism is the norm and not the exception (Aronin and Singleton 2008). It is not surprising, therefore, that much has been written about multilingualism, especially since around 1990. Before the new century began, the tendency in research was to separate theories of bilingualism and trilingualism. The boundaries were clear and unobscured. Trilingualism was relatively under-researched compared to the volumes of international research on bilingualism that emerged from the 1970s. It was in 2003, at Munster Technological University in Tralee, Ireland, where I now write this Foreword, that the third international conference on trilingualism was held, and here that the fact that research studies on bilingualism and trilingualism had started to morph into studies of multilingualism began to be heralded. The International Association of Multilingualism was founded at this conference and the International Journal of Multilingualism thereafter became the arena for a research focus on multilingualism, as distinct from trilingualism. The research agenda appeared to have shifted from trilingualism to multilingualism and this trend continues. While many authors, like De Houwer and Ortega (2018) and Baker and Wright (2017), for example, acknowledge the existence of trilingualism, the discipline now falls more and more under the umbrella of bilingualism or multilingualism, meaning the acquisition of more than two languages or language varieties. There was a growing realisation that multilingualism was not exceptional but rather ‘a normal and unremarkable necessity for the majority in the world’ (Edwards 1994: 1).

Since the conference, many research studies have renewed our understanding of multilingualism but there is still a need for further work. More and more theories of language are needed to explain and account for the day-to-day and real-world practices of multilinguals, including borrowing, transfer, mixing and translanguaging. More research is needed, particularly on the challenges posed by multilingual education, and especially in the design of curricula, curricular materials and pedagogical strategies. Nonetheless, since the turn of the millennium, research theories and approaches have become more dynamic and fast-changing. The concept and practice of translanguaging among bilinguals and multilinguals, in particular, is receiving increased attention in research, although its meaning is still developing. There has also been a growing number of studies on the extent to which language policies support or hinder societal and individual, bilingual and multilingual development.