An update of the chapter on preventing obesity in the SBU Report "Treating and Preventing Obesity – An Evidence Based Review". (Originally published in Swedish, 2002. English edition published in 2004.) Overweight and obesity are growing problems in much of the world. In Sweden, the number of overweight individuals has nearly doubled during the past 20 years. Obesity negatively affects people’s health and their quality of life. Finding strategies to overcome this problem is important for the well-being of both the individual and society at large. Obesity, once established, is difficult to treat, and therefore effective preventive interventions are essential. SBU has updated the report “Treating and Preventing Obesity” (2002) to include works published from 2001 through 2004 that address the prevention of obesity in children / adolescents and adults. The questions investigated include: How many studies of acceptable quality have been added to the literature? What do the new findings show? What are the combined results from previous and new studies? How do the results of the SBU review compare with those from other recent, systematic literature reviews? Are there important, preliminary research results that can be useful in formulating strategies for society, even if these results may not be fully verified by scientific standards? Since the release of the previous SBU report, the incidence of obesity has continued to increase at an unabated rate, representing a threat to the health and quality of life of the population. Obesity, once established, is difficult to treat. Hence, effective preventive interventions are essential. The following sections from the previous SBU report have been updated. The effects of intervention programs to prevent obesity in children and adolescents are relatively well studied. Ten works have appeared since publication of the previous SBU report, bringing the total to 24 studies involving nearly 26,000 individuals. Adding the studies found in other systematic literature reviews, but not included in the SBU review, brings the total to 39 studies and 34,000 individuals. From Sweden, only preliminary data are available from an ongoing trial. The consequences of relatively limited interventions lasting one or more years have been studied. Interventions have been based on programs in schools, day care centers, and other settings to increase physical activity and develop good dietary habits. Usually, a full package of interventions has been used. Based on the study descriptions alone, it is not possible to identify the interventions that have the greatest potential for favorable effects. Based on the studies in the SBU literature review, the evidence shows that school-based interventions can reduce weight gain and the development of obesity in children and adolescents (strong scientific evidence). Studies from literature reviews performed in other countries further support this conclusion. The fact that two thirds of the studies fail to demonstrate a positive effect may reflect the difficulty of achieving lifestyle changes in children and adolescents with school-based interventions alone that do not include the home environment, free time, and the community at large. The extent to which programs that are more long-term and comprehensive (, many types of interventions in the community) could be more effective has not been investigated. Extensive changes in society do not facilitate the use of control groups. Effects must be monitored through a reliable registry of weight trends in the population, not least trends among children and adolescents. Since publication of the previous SBU report, new studies have appeared on the prevention of obesity as one of several risk factors for cardiovascular disease in a normal population. Also there are new studies analyzing effects in individuals with an elevated risk for cardiovascular disease. Totally, the SBU reports now include 31 studies that involve nearly 64,000 participants. Interventions to improve lifestyle have included counseling on diets that are low in energy and fat and high in fiber. As a rule, recommendations to increase physical activity have been included, and several studies recommend smoking cessation and lower consumption of alcohol. The previous report presented 11 comprehensive, population-based studies, mainly dominated by North American studies. Recently, another 5 studies have been added. Of these, 2 are Norwegian and 1 is a Swedish study implemented in smaller regions, but with a longer time frame (5 to 6 years). Favorable weight trends were achieved in half of these studies, which was a better result than earlier reported. The findings give reason for greater optimism concerning the potential to prevent obesity in the adult population. Involving adults is also important considering their influence on the next generation. Special groups that are now being studied in normal populations include, , employees in companies or individuals in age groups at risk. Most of the studies show that weight gain can be prevented in these groups (limited scientific evidence). Interventions have had a positive effect on mean BMI in most of the studies aimed at individuals with an elevated risk for cardiovascular disease. Based on all of the studies involving adults, obesity can be prevented by interventions that improve diet and physical activity (moderately strong scientific evidence). The lack of effects in nearly half of the studies may be explained by the difficulty in achieving lifestyle changes, that the interventions have been too limited or they were limited to recommendations focusing only on increased physical activity.