This PhD thesis explored the health implications and socio-ecological determinants of sedentary behaviour. Specifically, it examined: 1) the associations between self-reported leisure-time and occupational sitting with cardiometabolic health risk factors; 2) the associations between work and leisure sitting with cardiorespiratory and muscular fitness; and 3) how housing characteristics and occupation type relate to levels of sitting. The results suggest that leisure-time sitting, rather than occupational sitting, is more adversely associated with health markers. Additionally, housing characteristics like household size were found to influence leisure-time sitting levels.