This thesis consists of two sections: section I - mode of birth delivery and asthma in childhood; and section II - sleep duration and obesity in adolescents. Section I - Prevalence of paediatric asthma has increased significantly over the past two decades. Parallel to this increase, rates of caesarean section have also risen considerably. Current updates made to the National Institute for Health and Care Excellence guidelines facilitate the uptake of elective caesarean sections which may impact on the prevalence of childhood asthma in the UK. This section compromises; (I) a systematic review and meta-analysis of the association between caesarean section and asthma in childhood. A 23% increase in •the subsequent risk of asthma in children delivered by caesarean section was shown; (II) a study investigating the association between the different methods of delivery and the development of childhood asthma was conducted through the use of birth and childhood data from the UK Millennium Cohort. A consistent association between caesarean section, asthma and its symptoms was shown. Section II - The rise in childhood and adolescent obesity occurred contemporaneously with a / decrease in usual hours of sleep, thus suggesting a potential link. This section compromise; (Ill) a study investigating the association between sleep duration and weight related parameters in Northern Irish adolescents through the use of cross-sectional data from the Young Hearts 2000 study . . A strong inverse association was found between average sleep duration and all weight related parameters; (IV) a study investigating the association between sleep duration and weight related parameters in Malaysian adolescents was conducted through the use of prospective cohort data from the MyHearts study. Inverse association was found between average sleep duration and most of the weight related parameters investigatedEThOS - Electronic Theses Online ServiceGBUnited Kingdo