Public health generally draws more on the medical, nutritional and physical sciences than the behavioural sciences to inform its policies and guidance. This article discusses the contribution recent behavioural science offers to the understanding of why people develop, and persist with, behaviours that are adverse to their health. Furthermore, it describes an online behavioural inter- vention that addresses people's habits, rather than their knowledge, to bring about sustainable improvements in health.Government attempts to change health behaviour have ranged from pro- viding information, incentives and overt persuasion, to regulatory measures. Yet, existing behaviour change interven- tions1 remain heavily predicated on an information deficit model, assuming some or all of the following:1. People lack knowledge about how to live a healthy life;2. People need educating about health;3. People will act in their own best inter- ests once educated.The obesity crisis demonstrates that educating people about healthy eating does not guarantee better dietary habits. In fact, despite the preponderance of health information in the media, in schools, on packaging, in health centres and in doctors' surgeries, obesity rates continue to rise.PERSISTENT PRE- EXISTING BEHAVIOURSFrom a behavioural science perspective, the failure of health messages to make an impact is due to the habitual nature of human behaviour. It is the entrenched nature of people's pre-existing lifestyle habits that renders them resistant to change. Although a minority of people will heed health advice and change their behaviour because it is good for them, others will have the intention to change, and under- stand the need to change, but will none- theless persist with their pre-existing behaviours. We focus on three reasons for this.One concerns the neural substrates of human cognition within a brain designed to automate all manner of choices and decisions rapidly, without conscious awareness. This results in behaviour being produced habitually rather than mindfully, even in the face of the person's attempt to exert self-control. Studies show that willpower is a limited resource that depletes rapidly.2 Nonetheless, many health interventions such as the National Health Service (NHS) Quit Smoking kit rely on the individual's ability to deploy willpower.3 Most people cannot use will- power to prevent habitual behaviour, as demonstrated by the failure of even a cancer diagnosis to motivate many to quit smoking.4Second, an individual's habit associa- tions are cued by their everyday environ- ment.5 An example is the case of Amy, a 36-year-old woman with a body mass index (BMI) of 38, for whom sitting on the sofa to watch TV with a cup of tea and biscuit is a regular daily occurrence. The sight of the sofa activates a habit chain in Amy's brain, associating the tea with the television and ultimately with the bis- cuits. Sitting on the sofa triggers a Pavlovian expectation of biscuits. The sound of the TV theme tune further stim- ulates Amy's brain into a state of expec- tation. The taste of her tea cues her eat- ing the biscuit. This context cueing, involving sensory activation, expectation and reward in the brain, is extraordinarily potent. It undermines any motivation Amy had to eat more healthily, thus fur- ther depleting her willpower and regula- tory control mechanisms.The third reason is the disconnect between knowledge and behaviour, or the knowing-doing gap.6 Neuroscientists know the human brain registers an action seconds before the thought of acting even enters consciousness.7 The conscious deliberating part of the brain barely impinges upon the unconscious, habitual behaviour drivers. Knowing and doing are frequently disconnected - yet health educators continue to expect understanding to lead to implementation.SUCCESSFUL BEHAVIOUR CHANGE INTERVENTIONS - 'DO SOMETHING DIFFERENT'Given these powerful brain mechanisms and the ability of context to trigger habitual responding, how can health interventions succeed? …