Cardiovascular diseases (CVD) are the leading cause of non-communicable deaths worldwide. It is well established that atherosclerosis, as a precursor to overt CVD, originates in youth. Vascular endothelial function has been identified as a ‘novel’ risk factor for CVD and can be non-invasively assessed using flow-mediated dilation (FMD). It has been shown that exercise improves vascular function, however, children and adolescents fail to meet the recommended daily physical activity levels. Therefore, it is important to identify the optimal exercise strategy to improve vascular function. In adults, new vascular measurements such as low-flow-mediated constriction (L-FMC) have been established to supplement the information gained from the measurement of FMD. Furthermore, the near-infrared spectroscopy (NIRS)-derived reperfusion rate of tissue oxygen saturation (slope 2 StO2) has been shown to be reliable and significantly correlated with FMD in adults, suggesting it may have utility to assess vascular function. Therefore, Chapters 4 and 5 are concerned with acute and chronic exercise, respectively, while Chapters 6 and 7 investigate novel assessments of vascular function and their reliability in a paediatric population. Chapter 4 demonstrates that both macro- and microvascular function were enhanced following an acute bout of interval running independent of exercise intensity in a group of adolescents, suggesting that the exercise pattern (i.e. interval) and the accompanying shear are important for vascular improvements. In addition, an acute glucose load improved macrovascular function whereas no changes in FMD were observed with a prior bout of exercise. The effect of a chronic exercise intervention on traditional and novel CVD risk factors in adolescent boys was examined in Chapter 5. Four weeks of high-intensity interval training improved FMD significantly with no concomitant changes in microvascular function or traditional risk factors (glucose, insulin, blood lipids, body composition), suggesting the training duration was too short to elicit further enhancements. However, the improvements in FMD were lost two weeks after training cessation. Chapter 6 showed that the brachial artery of adolescents constricts during a period of low flow, however, on an individual level low-flow vasoreactivity is inconsistent between days. The measurement of L-FMC has poor reliability compared to FMD and no significant correlation was observed between FMD and L-FMC. Finally, Chapter 7 revealed that the reliability of the NIRS-derived slope 2 StO2 is comparable to the reproducibility of FMD in male adolescents. However, slope 2 StO2 was not significantly correlated with FMD and therefore its use as a surrogate measure for FMD is precluded. In contrast to FMD, slope 2 StO2 was not sensitive to an acute bout of high-intensity interval exercise. In conclusion, this thesis demonstrates that the intermittent stimulus may be more important than the exercise intensity with regards to improvements in vascular function. In addition, enhancements of traditional CVD risk factors are not achievable within four weeks of training in healthy adolescents despite improvements in FMD. Improvements in vascular function were lost after two weeks of detraining, highlighting the importance of regular exercise for promoting vascular health. While the reproducibility of L-FMC in adolescents is poor, slope 2 StO2 can be measured reliably in adolescents. However, due to the lack of correlation with FMD, slope 2 StO2 may not be used as a measure of macrovascular function.