Levels of Physical Activity, Physical Fitness and Overweight/Obesity in Children and Adolescents Luísa Maria Seara Moreira Carneiro Aires Superviser: Professor Doutor Jorge Augusto Pinto Silva Mota Porto, 2009 This work was developed in the Research Centre in Physical Activity, Health and Leisure, CIAFEL, Faculty of Sports, University of Porto, Investigation Unit of Fundação para a Ciência e Tecnologia The present dissertation was written to obtain the title of Doctor in the Doctoral Course in Physical Activity, and Health organized by CIAFEL This work was supported by Fundação para a Ciência e Tecnologia SFRH/BD/23128/2005