Why have citizens become increasingly polarised? One answer is that there is increasing identification with political parties – a process known as partisanship. This chapter focuses on the role that social identity and partisanship play in contemporary politics. Partisan identities influence political preferences, such that partisans are more likely to agree with policies that were endorsed by their political party, regardless of the policy content, and, in some cases, their own ideological beliefs. We will describe how partisanship mirrors other forms of social identity, both behaviourally and in the brain. However, partisanship also has distinct biological origins, and consequences in political domains such as fake news sharing, conspiracy theory beliefs, and voting behaviour. Our chapter focuses on the psychology and neuroscience of partisanship within broader sociopolitical contexts.