Vaccination is regarded by the medical profession as one of the greatest public health success stories, and recent opposition, for example over the MMR (measles, mumps and rubella) vaccine, as a failure of understanding. Relatively little social scientific analysis exists on vaccination opposition. However, risk, trust and science are dominant themes within literature on public resistance to technology, and in contemporary theories such as risk society. This thesis therefore evaluates the relevance of these themes for an understanding of vaccination resistance in the UK. The empirical research primarily involves a discourse analysis of interview, document and website data generated from ten parental organisations, established to campaign against aspects of vaccination policy. The study defines these organisations as 'Vaccine Critical groups' and further classifies them into Radical and Reformist categories. In contrast to smallpox vaccination in nineteenth century England, vaccination is no longer compulsory in the UK. Nevertheless, from a governmentality perspective, the individual is still subjected to, what can be termed, the 'imperative of vaccination'. This thesis argues that the Vaccine Critical groups resist this imperative: first, by reframing risk as unknown, non-objective and individual specific; second, by demonstrating an ambivalent relationship with science; and third, by challenging faith in professional expertise and constructing the parent as the potential vaccine expert. These discourses create another type of moral imperative, which actually conforms to developments in the new public health that are encouraged by the state and the medical profession. The findings demonstrate the limits of a realist approach to risk, challenge existing theories of risk society and complicate assumptions about a public crisis of trust in expertise or science. Policy implications include the need to engage with vaccine resisters and their critical discourses, and to reassess the value of risk communication strategies.