This collection of essays examines the multi-faceted roles of experts and expertise in and around contemporary legal and regulatory cultures. The essays illustrate the complexity intrinsic to the production and use of expert knowledge, particularly during transition from specialist communities to other domains such as policy formulation, regulatory standard setting and litigation. Several themes pervade the collection. These include the need to recognize that: expert knowledge and opinion is often complex, controversial and contested; there are no simple criteria for resolving disagreements between experts; appeals to 'objectivity' and 'impartiality' tend to be rhetorical rather than analytical; contests in expertise are frequently episodes in larger campaigns; there are many different models of expertise and knowledge; processes designed to deal with expert knowledge are unavoidably political; questions around who is an expert and what should count as expertise are not always self-evident; and the evidence rarely 'speaks for itself'.