The problem of the worldwide rising prevalence of obesity as well as the rising physical deficiencies in children and adolescents should be kept clearly in mind. This is why it is important to counter this shocking trend and encourage the young people to be physically active and antagonize overweight. In order to achieve this aim, parents grant an important key function in many parts of children?s lives. Apart from health problems, overweight and obese people also struggle with discrimination and stigmatization which negatively affects people?s quality of life.Considering this, my paper aims to analyze the relationship between physical fitness of children and adolescents, their relative body-mass-index (BMI-SDS) and motivation to exercise. Furthermore, the examination of parental influence on their children?s physical fitness, the survey of predictors for quality of life and the assessment of changes in BMI-SDS and physical fitness over one year were conducted.The two samples are composed of 26 children (10 male, 16 female; average age 11.15) and 21 adolescents (10 male, 11 female; average age 10.48).Statistical methods applied were univariate variance analyses with repeated measures, multivariate variance analyses, t-tests, Pearson correlations and multiple-regression-analyses.Results show a positive correlation of physical fitness of children with their BMI-SDS, their motivation as well as their parents? motivation to exercise. Furthermore, motivation to exercise demonstrates the most important influence on quality of life. Additionally, my thesis proves that physical fitness increased over time.In conclusion it can be stated that physical fitness is an important factor influencing bodyweight positively and that parents? exemplification of an active lifestyle mainly accounts for an evenly active lifestyle of their children.