Chapter 3 centres on the ways knowledges are used in policy processes. Knowledge can be used instrumentally where it is applied to a policy problem. Here the knowledge or evidence is seen as agreed upon and uncontroversial. Knowledge can be used politically to support a policy position. It can also influence policy conceptually. This chapter explores how knowledge is used instrumentally, politically, and conceptually in behavioural insights, building a clear and balanced analytical model that follows these uses. The chapter demonstrates the importance of studying dynamic webs of knowledge in behavioural insights, as knowledge uses are not separate but interwoven and mutually reinforcing.