This chapter defines “expert consensus in science” as a high level of agreement among scientists with relevant expertise about a specific scientific claim, methodology or science-based practice or policy. In the not-too-distant past, the topic of expert consensus in science would only have been of interest to scientists and scholars in the history, philosophy and sociology of science. However, there is now a much broader interest in the topic from both scientists and non-scientists alike. A major reason for this is that expert consensus is increasingly used to guide national policies based on scientific findings, but some of these policies may be at odds with people’s pre-existing beliefs and values, and they understand little about scientific consensus. Examples are the role of human activity in global warming, the use of genetically modified crops and the safety of vaccination of infants. The chapter surveys the range of sceptical opinion about the legitimate role of consensus in science. The book challenges the sceptical view, arguing that expert consensus plays a vital role in scientific processes. However, these processes could be improved, and the book proposes how this could be done.