Cardiovascular disease (CVD) is one the greatest causes of death in Western societies, and it probably will be also in the future in developing countries. Programs aiming at the prevention of CVD mostly focus on the development of a “healthy” lifestyle. Physical activity is considered to be an important component of such “healthy” lifestyle. Besides physical inactivity, also low physical fitness is found to be related to many chronic diseases, but probably the best evidence has been found for CVD [ 5 ] [ 11 ] [ 15 ]. The importance of habitual physical activity and physical fitness is not only reflected by the relative risk of physical inactivity and low physical fitness for CVD, but also by the high prevalence of physical inactivity and low physical fitness in Western society. For instance, the population attributable risk (PAR) of physical inactivity for CVD mortality is 35 % [ 18 ]. This means that 35 % of the deaths caused by cardiovascular disease could theoretically be prevented if everyone were sufficiently physically active. Based on recent population data in the Netherlands, PARs of different lifestyle related parameters were calculated for cardiovascular disease mortality [ 19 ] [ 20 ]. The PAR for a sedentary lifestyle was of the same magnitude as the PAR for smoking (± 40 %) and much higher than the PAR for obesity (± 14 %). Even though the clinical symptoms of chronic diseases do not become apparent until much later in life, it is known that the origin of many chronic diseases lies in early childhood. It is therefore often argued that prevention of chronic diseases has to start as early in life as possible [ 13 ]. The fact that many chronic diseases start early in life is now well accepted mainly due to the work of Barker [ 2 ] [ 3 ] [ 4 ] who hypothesised that the basis of many chronic diseases is formed in foetal life. In other words there is nowadays great interest in the investigation of early development and health related outcomes. There is, however, not much information available on the relationship between the early development of physical activity and physical fitness on health outcomes later in life. The papers of this supplement aim to fill this gap in the literature, and although the early development investigated does not go as far as foetal life or infant stage, we still feel that the information provided in this supplement is of great public health importance. With regard to physical activity, especially the adolescent period seems to be important. It is known that in the Western world the amount of habitual physical activity is decreasing enormously in this age period [ 1 ] [ 17 ]. There are three possible pathways through which physical activity and physical fitness can influence cardiovascular health (see Fig. [ 1 ]). First of all, there is a possible direct relationship between physical activity and physical fitness during youth and CVD-health at adult age. Furthermore there are two indirect possible pathways: 1) Physical activity and physical fitness during youth are related to health status during youth. This can be important because from the literature it is known that health status during youth is an important predictor for health status at adult age [ 13 ] [ 21 ] [ 22 ]. 2) Physical activity and physical fitness during youth are related to physical activity and physical fitness during adult age. This can be important because there is extensive evidence that physical activity during adult age is related to health status at adult age [ 8 ] [ 11 ] [ 18 ]. Fig. 1 Different pathways through which physical fitness and physical activity during youth can influence health outcomes at adult age. The ideal study to answer the question whether high levels of physical activity and physical fitness during childhood and adolescence lower the risk of developing CVD later in life is a randomised controlled trial with a lifetime follow-up, in which a large group of children and adolescents is assigned to either a sedentary or an active lifestyle; a study which will probably never take place. Second best to such an experimental design is a long-lasting observational study in which both lifestyle (including physical activity behaviour) and other risk factors for chronic disease are measured repeatedly over time. Preferably such a study should already start at an early age in life and CVD mortality or morbidity should serve as outcome. The most classical, and probably the only study investigating the relationship between physical activity in relatively young people and the occurrence of CVD mortality at later age is the Harvard Allumni Study, performed by Paffenbarger et al. [ 16 ]. In this extensive and long lasting observational study physical activity levels during the student period (gathered from university archives) were related prospectively to the occurrence of CVD later in life. Regarding their (baseline) physical activity levels, the students were divided into three groups: 1) athletes, 2) intramural sports play for more than five hours per week, and 3) intramural sports play for less than five hours (usually none at all) per week. These three groups did not differ regarding the occurrence of CVD later in life. The study showed that student athletes who had discontinued their activity levels after college encountered a CVD incidence similar to the risk of alumni classmates who never had been athletes. The study also showed that the subjects who became physically active later in life had the same health benefits as the subjects who had been active all along the observation period. These findings suggest that the effect of physical activity on cardiovascular health disappears over a period of 25 years, so maybe the effects of physical activity are short lasting. However, due to the lack of studies, it is not really clear how much time it takes before the effects of physical activity are wasted after...