This volume is the first of a two-volume collection on mental states. The contributions to this volume focus on evolutionary and functional aspects of certain mental states in an effort to understand their nature, whereas the second volume is concerned with the question what language and language use reveals about cognitive structure and underlying cognitive categories. Questions that are addressed in this volume include: (i) how early did cognitive states of a sort rich enough to support communication and planning appear in the evolutionary history of hominids?; (ii) is it possible to infer the existence of sophisticated cognitive states from evidence of tool use?; (iii) how do mental states represent situations or events or actions?; (iv) how can we theoretically model mental states?; (v) how can we simulate mental states and their functions?; and (vi) what insights can conceptual categorisation - both linguistic and non-linguistic - give us into the organisation and structure of the mind and hence of mental states? The volume contains contributions from psychologists, linguists, artificial intelligence researchers, neuroscientists, archaeologists and philosophers, bringing together scholars from the diverse fields of cognitive science, or more specifically, the study of language and cognition. This reflects the provenance of the chapters, most of which were presented at the 'International Language and Cognition Conference', held in September 2004 at Pacific Bay Resort in Coffs Harbour, Australia.