Worldwide obesity has more than doubled in the last forty years. Even more worrying is the fact that the number of overweight and obese children and adolescents has considerably increased. Socioeconomic development, as well as educational, agricultural and marketing policies have significantly changed dietary and physical activity habits among the youngest, who are thus susceptible to develop chronic and disabling diseases such as diabetes, some cancers and cardiovascular disorders. Adolescence is a critical age, in which the adoption of healthy habits may have dramatic effects on the health state in adulthood. For this reason, prompt interventions are urgently required to prevent the onset of obesity in this time of life. In this regard, the CAL-TAS program from Alicia Foundation was born to combat obesity and promote healthy lifestyles in Spanish adolescents. A total of 2519 students, aged 13-14 years, from 79 schools distributed all over the 17 autonomous communities in Spain were asked to report through the CAL-TAS platform their food intake and physical activity over one week. The body mass index, the consumption of food and beverages, the intake of macronutrients and micronutrients, and the values obtained from the PAQ-A questionnaire, which evaluated physical activity, were analyzed. Twenty percent of the participants were overweight or obese. In general, adolescents did not or poorly respected the recommendations provided by the Spanish Society of Community Nutrition. For example, in more than half of the subjects, the ingestion of fruits and beverages was less than recommended, whereas the consumption of meat, baked goods and fried foods was excessive. Moreover, adolescents with higher body mass index also presented worse eating habits and more inactivity. In conclusion, Spanish adolescents present low adherence to recommendations provided by the Spanish Society of Community Nutrition (SENC) and by the World Health Organization. In order to prevent obesity-related disorders, effective educational programmes have to be designed. Indeed, adolescents and their families should be aware that the early adoption of healthy dietary habits and of a correct physical activity may strongly improve their future quality of life.