Hospitals are full of posters telling people to wash their hands, and full of staff who already know they should. Patients miss appointments, staff work around cumbersome systems, targets distort behaviour and well-intentioned policies produce unintended consequences. Much of what goes wrong is shaped by human behaviour. Drawing on behavioural science and real-world experience in the NHS, this book argues that outcomes depend on the environments in which decisions are made. Rather than a catalogue of ‘cognitive biases’ or ‘nudges’, it offers a practical framework for improving healthcare at every level, from patients and clinicians to managers and policymakers. Small changes to system design can have large consequences, often without waiting for medical breakthroughs, new technology or more funding.