Prevalence of obesity in Korean children and adolescents has dramatically increased since the last 10-20 years. It is important to initiate prevention efforts early in childhood because prevalence of obesity in adolescence is the strongest predictor of its prevalence in adulthood. Intrauterine life, infancy, and preschool years may comprise the critical periods that are essential for the long-term regulation of energy balance therefore, obesity-prevention strategies should be initiated in utero and continued throughout childhood and adolescence. Families with high-risk children should be provided early education about maintaining normal weight. Encouraging physical activity and, especially, avoiding inactivity, are key challenges in the prevention of future obesity. Schools should be primarily involved in educating parents to discourage their children from excessively watching TV or playing computer games and eating unhealthy snacks and food. The involvement of medical practitioners is also important, especially, in the case of obese parents, obesity prevention strategies should be promoted from the first visit of pregnant women to the physicians. Health professionals can also be involved in obesity prevention because they are ideally equipped to identify young children at risk of obesity. Community and nation-wide efforts to increase awareness and promote environments that encourage physical activity and healthy nutrition are required.