The three aims of the series of studies presented in this thesis were to investigate physical activity (PA) in childhood, the associations between PA and cardiometabolic (CM) risk in childhood, and interventions to promote PA in childhood. Study 1 investigated the relationships between invasive and non-invasive markers of CM risk, and also examined the associations between individual CM risk markers and PA, cardiorespiratory fitness (CRF) and body composition in a cohort of school children (n = 101, 10 - 11.9 years) from Liverpool, England, and Belfast, Northern Ireland (NI). An individually calibrated approach to PA measurement was used to improve the classification of children's PA, and to clarify the associations between PA and markers of CM risk in children. Reference standard measurement techniques were used to assess body composition, PA and CRF. This range of high-quality objective measures is rarely observed in similar crosssectional studies with children. Few studies investigating CM risk in youth have included functional/pre-clinical' markers of CM risk, such as flow mediated dilation (FMD) and carotid intima media thickness (CIMT), or markers of inflammation such as adiponectin and C-reactive protein (CRP). Furthermore, few studies have investigated the associations between these preclinical markers of CM risk and invasive (inflammatory and metabolic) markers of CM risk in children. The inclusion of these measures enabled an investigation of the relationships between functional, metabolic and inflammatory components of CM risk in children. In Study 1, moderate to vigorous physical activity (MVPA) was positively associated with CRF in boys and girls, however, the associations between CRF and CM risk were higher than those observed between PA and CM risk. The associations between, PA, CRF, sedentary time (SED) and CM risk were often mediated by adiposity. The percentage of 10 - 11 year old boys (30%) and girls (22%) meeting current guidelines for daily MVPA was low. This study suggests that adiposity represents a suitable primary outcome measure to evaluate the effectiveness of interventions to improve CM health in children. Furthermore, as the associations between CRF and CM risk were higher than those observed between PA and CM risk, and a strong body of evidence suggests that vigorous PA is required to improve CRF in children, interventions to promote CM health in children should emphasise vigorous physical activities. This study also highlights the need for interventions to increase PA in primary school children.