Many researchers have observed the paradox of the decline in trust of experts alongside the increasing use of expertise in Western society. This research argues for the division of expertise into a defined category, “expert-source”, separating experts from other sources who do not possess expertise. Using a normative concept of expertise to provide categories can offer a more coherent and consistent method of assessing a source's expertise and how to present their statements. This research presents findings based on a media analysis of television, radio and newspapers, interviews with journalists and sources and the results of national surveys and focus groups with parents and uses a recent medical controversy in the UK as a backdrop to explore expertise. In 1998 a scientist claimed there might be a link between the measles, mumps and rubella vaccine and autism. His claims received significant media attention and vaccination rates fell across the UK. This case study examines how journalists constructed expertise, how key sources presented themselves as expert-sources and the effect of balancing expert-sources with sources. This research encourages journalists and academics to question how expertise affects media coverage.