Expertise is undoubtedly a feature of policy areas in contemporary societies; inputs of formal knowledge are important components of the policy process. The post- 1945 period, in Britain, has witnessed a proliferation and diversification in both the number and type of expert, and expert group used in policy areas. This situation poses both theoretical and empirical difficulties for any analysis of the role and influence of expert groups. The thesis focuses upon expert groups in the policy process, the nature and reasons for their influence, and the subsequent democratic and normative implications raised by such an analysis. The thesis makes an assessment of the various definitions of the expert, as well as a thorough examination of technocracy. Moreover, the nature of expertise is examined to demonstrate how expert influence may alter at different levels in the policy-making process. It is contended that complexity, and a corresponding requirement of legitimacy, are the primary reasons for the use of experts in policy areas. This analysis is placed in historical context since 1945, a period that has witnessed alterations in both the nature and type of expert and expert group, the level of demand for expertise, and the reasons for that demand, made by policy- makers for expert input. This theoretical and historical overview is utilised in the analysis of two diverse case studies of policy sub-sectors: the case of air pollution and asthma, and the siting of the route of the Channel Tunnel Rail Link. Although distinct in nature, it is shown that within both of these policy areas expert groups operate on a formal mandate, as a result of complexity and a requirement of legitimacy. And furthermore, that the primary effect of the complex nature of the policy-making process, and the corresponding use of expert knowledge to legitimate policy, is on the level and type of political participation.