Methods/Design: The study design is a randomized controlled multicenter clinical trial using two types of treatment. Population and sample: Children 6 to 12 years with overweight or obesity, according to the z score of body mass index (BMI z) >= 1 for age and gender defined by International Obesity Task Force (IOTF). It is considered necessary to recruit 100 children: 50 control group (CG) and 50 Intervention (IG). Study Intervention: Nereu Group: 8 month intervention with 3 weekly training sessions doing physical exercise for children and a weekly session for parents of physical activity and healthy eating habits and behaviour strategies, that involves both parental and child participation. The control group will receive a monthly session of healthy physical and eating habits. Main Outcome Measures: Improvement of BMI z, physical activity and nutrition habits, behaviour components and quality life related to health at the end, 6 and 12 months after intervention.