The first part of this book provides an overview of the worldwide prevalence of overweight and obesity and its key determinants in children and adolescents. However, such a comprehensive overview is hampered by several limitations. (1) While in some countries large scale systematic surveys were conducted that provide data that are nationally representative, in many countries of the world no or only small surveys were conducted. Most data refer to restricted study populations who were selected by varying sampling schemes which makes a judgment of their representativeness difficult. (2) Data were obtained during differing years such that time trends may in some cases mix up with geographic variations.