The growing social and economic significance of expertise is reflected in suggestions that we are moving into a post-industrial "knowledge society". The subject of expertise is becoming recognized in a range of disciplines ranging from science and technology, psychology, computing and artificial intelligence through to management and organizational behaviour. This work brings together some of the diverse understandings of the character and implications of expertise, and demonstrates through a set of empirical case studies how expertise means different things to different groups, how it is constructed differently in different settings, and the consequences of this process for relations between "members" of the knowledge society and those "on the outside". Case study material ranges from a hospital ward and a factory, to a nuclear weapons facility.