This Supplementary Anniversary Issue is a gift to the child language community to celebrate the fortieth anniversary of the Journal of Child Language. The first issue appeared forty years ago, in May 1974. In his guest Editorial, David Crystal, the first editor of JCL, describes how the newly emerging field was in need of a specialized journal. The field is still booming: JCL started with two issues in 1974 and 1975, expanded to three issues in 1976, and to four issues in 2002. In 2006, the electronic submission system was introduced. 2009 was the first volume with five issues, and in 2014 we moved to six issues. The online ahead of print publication (First View) was another important step in the dissemination of child language research.

This Anniversary Issue is a ‘Festschrift’ or homage to the field, written by scholars who helped to shape the field and substantially contributed to the Journal of Child Language – as author, reviewer, Editorial Board member or (Associate) Editor. Together, the authors have more than 500 years of experience in studying language acquisition, and they have promoted, refined, and rethought many of the issues that need to be solved. The authors were asked to contribute a reflection piece about the past and the future of the field, and were free to choose their own focus rather than respond to a fixed set of questions or themes. The result is a multitude of perspectives: historical, theoretical, and programmatic.

David Crystal's guest Editorial provides a vivid account of the founding stages of the journal, which he edited for the first eleven years. Based on a language count in the titles, he argues that still four-fifths of what we know about language acquisition is based on English, a point also raised by Ruth Berman and Dan Slobin.

Dan Slobin reviews the technological advances in studying child language data, and how they allowed researchers to investigate new questions. However, he argues, in addition to advances in recording technology and experimental procedures, linguistic diversity itself is a research tool. The systematic consideration of typological differences can help us discover which cues children use to form their form−function mappings.

Paul Fletcher demonstrates how the CHILDES database and the CDI (Communicative Development Inventory) for lexical and early grammatical development contribute to the precision and reliability of child language research. These tools are now well-established resources for data-sharing and for developing standardized and transparent procedures for the analysis of language development at all levels and across languages and their varieties.

The rich databases available today allow us to investigate long-standing debates from new perspectives: Elena Lieven argues that computational analyses of large datasets provide evidence for several layers of linguistic units that children generalize. Some of children's error patterns as well as their development of productivity can be explained from the chunks or form−function units children encounter in their language input.

Ruth Berman reviews the history of cross-linguistic corpora for studying universal and language-specific processes in children's development from proficient to native speakers. Systematic cross-linguistic comparisons of children's narratives provide evidence that the semantic categories children develop are shaped by the language-specific form−function mappings.

Laurence Leonard shows that children with atypical language development provide yet another window into the interaction of factors like genetics, age, input properties, and the processing factors needed to extract grammatical relations from speech. The problem is to account for the differences in which different populations extract information from the input.

This issue is also addressed by Susan Goldin-Meadow. She analyses which properties of language are quite robust or resilient in language development, and what causes other aspects to be more fragile. The study of sign languages that emerge from homesign may be a window to analyze which variations in the language learning environment lead to variations in the outcome, and which properties are robust and thus quite unaffected by variation.

Virginia Valian discusses the difference of properties that make language special, and those that may be innate. She proposes that the determiner system is an innate abstract theory that enables children to productively acquire the skeletal structure of NPs, and that this may constitute convincing and converging evidence for the existence of an innate abstract syntactic feature.

Several contributions look at the acquisition of meaning through interaction. Katherine Nelson argues that we need to go beyond simple form–function mappings that focus on the referential function of words, notably nouns. There are wide-ranging individual differences: not only do children differ widely in the age of onset and speed of vocabulary acquisition, but they also differ in the functions that the words encode. While some children learn an array of object words, others first focus on words and phrases that encode their needs, interests, and feelings.

Eve Clark discusses the socio-pragmatic prerequisites for successful communication, like joint attention and the interactive establishment of common ground, as well as the often very subtle feedback and repair mechanisms that help children in refining their language use. Recent research in (developmental) psychology and linguistics has shown how children use intention reading in their social coordination with others.

Catherine Snow also looks at interaction, but with respect to the qualitative and quantitative aspects of input and its influence on language development, not just in the early phases, but in particular regarding the acquisition of the complexities of academic language. She argues that in order to prepare children for school and professional life, we must analyze the social and cultural variation in academic language, and work out the relationship between input and interaction on thinking, literacy, and academic outcomes. The results of such research would enable us to design teaching and intervention measures.

Finally, Brian MacWhinney takes on the task of defining the effect of learning principles, processing factors, neuronal bases, and social structures on language development in order to characterize the state of the art of the field. But he also identifies largely uncharted territory in those areas that emerge in larger timeframes, such as the semantics of abstract and theoretical constructs, or pragmatic skills like persuasion.

It seems that two themes for future research emerge from the opinion pieces presented here: first, we need to continue to assess the full range of variation regarding genetic, individual, typological, and societal factors that contribute to language learning in order to explain how these factors interact to produce the diversity of language use that we find in the world's languages. Second, research on the acquisition of the formal properties of language needs to be accompanied by research on their semantics and pragmatics to investigate how children differentiate their language use in order to encode subtle differences in meaning.

Of course, there is much more to say, more areas to be covered, and many more voices to be heard than can be represented in this small Anniversary Issue. The full history of the field remains to be written, but we hope that the insights and open issues presented in these reflection pieces will stimulate a vibrant discussion regarding uncharted territories and places to be revisited.

Forty years of JCL also provide an occasion to thank the previous editors: David Crystal acted as the founding editor from 1974 to 1985, and was assisted by Paul Fletcher and Michael Garman. Alan Cruttenden took over editorship in 1989, and was joined by Katharine Perera in 1989. Katharine Perera then acted as the sole editor from 1991 to 1997, when Elena Lieven assumed the post, which she held until 2006. Philip Dale and Edith Bavin edited the journal from 2006 to 2010 and 2111, respectively. They introduced me to the task, and Edith helped in shaping the idea and the structure for this Anniversary Issue.

In the past forty years, the following colleagues acted as Associate Editors and carried the legwork for the review process (in alphabetical order): Shanley Allen, Edith Bavin, Misha Becker, Heike Behrens, Glynn Collis, Katherine Demuth, Daniel Dinnson, Paul Fletcher, Michael Garman, Patrick Griffiths, Erika Hoff, Margaret Kehoe-Winkler, Aylin Küntay, Peter Lloyd, Letitia Naigles, Johanne Paradis, Katharina Perera, Ann Peters, Clifton Pye, Brian Richards, Caroline Rowland, Carol Stoel-Gammon, Stephanie Stokes, Holly Storkel, Rosemarie Tracy, Kamil Ud Deen, and Elizabeth Wonnacott. Glynn Collins also acted as a statistical advisor for several years. When JCL carried book reviews, Virginia Mueller Gathercole and Evan Kidd were the Book Review Editors.

The editorial team was and is supported in numerous and indispensible ways by the editorial assistants and the staff at Cambridge University Press. Our heartfelt thanks to them for their reliable and prompt solutions to all practical and technical problems.

But, of course, we could not run the journal without the members of our Editorial Board, who have supported the journal throughout, the numerous reviewers, who make the journal what it is, and most of all, our authors, without whom the journal would not exist. We are looking forward to your continued contributions for decades to come.