“Doing some physical activity is better than doing none.” (10 facts on physical activity, World Health Organization) While the benefits of physical activity have long been recognised, the threats of inactivity have been exacerbated by modernity. Not only have modern technological advances made it possible for less physical exertion to take place, but the general increase in life expectancy has also opened the flood gate for a series of chronic (non-communicable) diseases. Lack of physical activity detrimentally increases several risk factors for chronic disease and death, including raised blood levels of lipids, glucose, as well as high blood pressure. Inactive people are more likely to develop obesity, diabetes, cardiovascular disease (heart disease and stroke), osteoporosis and some cancers (breast and bowel), all of which pose major public health problems. Interestingly, evidence emerges concerning the link between low activity and a greater risk of dementia, depression and impaired physical function in the elderly. Convincing research suggests that sedentary behaviour has harmful health effects independent of physical activity, meaning that high levels of activity don’t cancel out the effects of sitting down for extended periods of time. Physical activity can be pursued in four ‘domains’ of daily life including leisure time, work, transport and at home. They act on health independently through different pathways. For my PhD research, I am using data from epidemiological studies led by the Medical Research Council (MRC) Epidemiology Unit in Cambridge and collaborators to accurately assess activity levels in adult populations in order to examine the interplay between different activity domains (especially leisure and work) and health. Another fascinating aspect on which I am trying to shed some light is the importance of the dimension of activity for a given health outcome. In short, what matters more: overall energy expenditure (total calories you burn) or time spent at various intensity levels (how hard you work). Some of the studies that I have drawn from are: EPIC, Fenland, InterAct and National Survey for Health and Development. The world-class expertise of the MRC Epidemiology Unit lies in its development of novel technologies for the objective assessment of physical activity, such as waterproof monitors that are easy to wear and collect information on heart rate and body acceleration. These devices have opened up new research avenues which enable a much more accurate estimation of activity than questionnaires. Building on these results, my work seeks to contribute to these studies which provide the high quality means for informing future public health policy and practice. The societal burden from physical inactivity is manifested as a growing healthcare budget and a loss of productivity. According to the World Health Organization (WHO), insufficient physical activity is the fourth leading risk factor for mortality and causes 6% of deaths worldwide. In 2008, there were 5.3 million deaths due to physical inactivity, which is equal to the number of deaths from smoking. Intriguingly, research has shown that a failure to spend 15-30 min/day in activity of moderate intensity shortens people's lifespan by three to five years. Global physical activity surveillance efforts based on questionnaires have shown that 31% of adults worldwide are physically inactive, highlighting the pandemic proportion of this behaviour. However, investigators on the National Health and Nutrition Examination Survey in the USA used accelerometers and showed that more than 95% of American adults fail to meet physical activity guidelines. With an aging global population, the elderly represent the fastest growing group, which is also the least active and has the highest risk for developing chronic disease. Therefore, promoting physical activity in later life is an important approach to help those individuals achieve healthful aging, thereby mitigating the risk of disease and perhaps even slowing the aging process. How much activity? How active do we have to be? According to the current guidelines by American Heart Association and American College of Sports Medicine, a minimum 30 minutes/day of moderate-intensity activity (e.g. brisk walking) five days per week, or 20 minutes of vigorous activity (e.g. jogging) three days per week, or a combination of the two should be performed to maintain health and reduce the risk of chronic disease. Sedentary behaviour should be reduced and activities to maintain or increase muscular strength and flexibility should also be performed at least two times a week. In order to prevent unhealthy weight gain, adults should engage in 60 minutes of moderate-to-vigorous activity every day. Those wishing to sustain weight loss need to participate in 60-90 minutes of moderate-to-vigorous activity daily without exceeding calorific intake. In the context of chronic disease prevention and health promotion, physical inactivity has not been given as much attention as other risk factors such as smoking, diet or alcohol. It is extremely important to stress that even small amounts of physical activity confer substantial benefits beyond cardiovascular and metabolic health, including improved mental wellbeing and quality of life as well as the positive effects on environment that result from replacing car use by cycling or walking. In the battle against inactivity, emphasising its harm is of paramount importance in addition to advocating the benefits of an active lifestyle. The global challenge is to make physical activity a public health priority in order to improve population health and decrease the burden of chronic disease. This aim clearly requires good quality research to provide evidence and inform public health policy, as well as ensuring that environments are safe and supportive of health and wellbeing through collaboration across all sectors of society including various stakeholders, such as health care, community planning, education authorities, nutrition, employers, transport engineers and academia. *Rajna Golubic  is doing a PhD in Epidemiology which she began in 2010. Picture credit: Sura Nualpradid and www.freedigitalphotos.net.