One of the most central themes in cognitive science concerns the relationship between language and the origin and development of concepts – a enduring topic [1xSee all References][1] that is being addressed increasingly in a surge of books, articles and commentaries (e.g. [2xThinking through language. Bloom, P and Keil, F.C. Mind Lang. 2001; 16: 351–367CrossrefSee all References, 3xSee all References, 4xThe faculty of language: what is it, who has it, and how did it evolve?. Hauser, M.D et al. Science. 2002; 298: 1569–1579Crossref | PubMed | Scopus (1732)See all References, 5xSee all References, 6xSee all References, 7xIs language the key to human intelligence?. Premack, D. Science. 2004; 303: 318–320Crossref | PubMed | Scopus (91)See all References]). This issue has often been viewed to be the key to understanding the distinctive conceptual basis of human cognition, with its flexibility and adaptability in reasoning and problem solving. The many interrelated questions that arise are daunting and complex: How much in the way of conceptual understanding occurs in the period of human development – infancy – in which there is not yet overt language? Does language enable us to reason efficiently about the properties of specific areas or domains of knowledge in the physical and mental world? For example, does it enable us to have a theory of mind in building a concept of true and false belief (in providing a syntactic structure that allows us to consider propositions such as ‘Jane believed [falsely] that John put the cookies in the cupboard’)? Does it allow us to navigate in the world and to find objects (in providing us with spatial terms to reason about location)? Does it allow us to carry out numerical computations (in providing us with the capacity to carry out mathematical operations based on recursive relations, parallel to the syntax of language)? Is it language that permits us to entertain propositions about causality, enabling us to have a scientific understanding that goes beyond correlations and superficial appearances, and in turn allowing us to understand causal mechanisms in biology, chemistry and physics?And what about those who are impoverished in language, and those who are abundantly endowed? Are deaf children without early access to a sign language impaired too in their conceptual development? What about children who are bilingual or even trilingual? Are their conceptual systems enriched in the process of knowledge acquisition compared with monolingual children? Moreover, even if language in the form of syntax and semantics does not uniformly provide a structure for conceptualizing the mental and physical world, could it be said that the development of intentional communication – pragmatics – is what bridges the gap between a conceptual understanding that is implicit in early thinking and its actual expression in reasoning and problem solving?Definitive answers to any one of these questions would bring us closer to understanding the essence of human cognition. Indeed, we now have research programs that hold out the prospect of determining what is special about language in adjudicating between accounts of its role in conceptual development within specific domains of knowledge. This work will be featured in TICS during 2004/5 as part of a special series of Review and Opinion articles on language and conceptual development (see Box). These state-of-the-art perspectives will focus on children's conceptual knowledge before and after the acquisition of language, in areas as diverse as number and biology, and will therefore illuminate the issue of how to characterize the relationship between language and conceptual development.