“Healthy City” initiatives are all the rage, but what, in the 21st century, makes a healthy city? Pre-industrial cities were frequently ravaged by famine and disease; this was exacerbated by intense overcrowding following the industrial revolution. At present, the biggest cause of preventable death in the developed world is heart disease, while rates of depression and mental illness have increased steadily since the 1970s (Frumkin et al, 2004). An oft-cited underlying reason for the recent changes is a lack of physical activity among a significant proportion of countries’ populations. The predominant policy response has mostly taken the form of campaigns to raise awareness among the population and encourage behavioural change. At a national level, both the UK Chief Medical Officer and the US Surgeon General advise 30 minutes of physical exercise per day. This advice, and accompanying policy, is bolstered by very comprehensive research.Such advices - and accompanying policies - have however met with limited success; sedentary activities have increased over the past decade. This is because many policies aim to deliver results by changing people’s behaviour. This is very difficult to do successfully; behavioural initiatives have to be targeted across a population, addressing very wide ranges of underlying behavioural reasons for physical inactivity. They also rely on a high degree of general self-discipline.Physical activity can be divided into two main types; recreational and utilitarian (Frank et al, 2003). Recreational activity is behavioural and is undertaken by virtue of its inherent pleasure - someone rarely goes rock climbing, for example, for reasons other than recreation. Utilitarian exercise, by contrast, is undertaken as part of some other task cycling to work, for example. Recreational activity is thus determined according to the preferences of the individual. Utilitarian activity is by contrast undertaken as a pragmatic response to the physical environment. This is of particular relevance to the design of the built environment with respect to public health and wellbeing. In this paper, I will describe and analyse the extensive body of research that has been taking place into the relationship between the form of the built environment, levels of physical activity and health. I then examine ways in which the form of the built environment – particularly walkability – can be objectively measured and positively correlated with levels of physical activity and thus health, irrespective of expressed preferences for exercise. This in turn can be used to draw up identifying characteristics of the types of urban form that are the most conducive to physical activity. This has significant policy implications for urban design and its potential for contributing to improved public health, social cohesion and environmental benefits.