In recent years, a wave of reforms known as nudges or behavioural interventions have emerged in public policy and administration. Nudge policies are created to lightly influence groups in society to change their behaviour, using behavioural insights to solve complex policy problems. Generally, behavioural approaches focus on the psychology underlying the implementation and effects of policies in practice. First published as a special issue of Policy & Politics journal, this book situates these reforms within a broader tradition of methodological individualism. With contributions from international scholars, it demonstrates that when behavioural policies expand their focus beyond the individual, they have the potential to better understand, investigate, and shape social outcomes.