Category-specific knowledge disorders are among the most intriguing and perplexing syndromes in cognitive neuropsychology. The past decade has witnessed increased interest in these disorders, due largely to a heightened appreciation of the profound implications that an understanding of concept representation has for such diverse topics as object recognition, the organisation of the lexicon, and storage of long-term memories. Until recently, information about the representation of concepts was limited to findings from patients with brain injury and disease. This state of affairs has now changed with the advent and wide-spread availability of functional imaging for studying cognition in the normal human brain. The purpose of this special issue is to provide a forum for new findings and critical, theoretical analyses of existing data from patient and functional brain imaging studies. The contributions, all from major investigators in the field, range from studies of specific object categories such as animals, tools, fruit and vegetables, and faces, to the more general domains of number processing, social interaction, and mechanical knowledge. A unifying theme of these papers is the extent to which the findings can be best understood within the context of models that posit an innate, domain-specific organisation, those that appeal to an organisation by sensory- and motor-based features and properties, and those that propose an undifferentiated, distributed neural organisation.