Worldwide, there is a growing burden of non-communicable diseases (NCDs). The rise has coincided with the increase in prevalence of overweight and obesity. The ANGELO framework recognises the role of environmental influences in weight gain. In Africa, various environmental influences have led to shifts from a traditional dietary patterns and more physical lifestyles to a “Western” dietary pattern and low physical activity. The life course perspective posits that excessive body weight can persist from adolescence to adulthood, and increase the risk of NCDs. Among adolescents the school environment is a crucial setting for the development of and, or engagement in unhealthy dietary and physical activity behaviour. The aim of this thesis was to investigate in the school environment, the consumption of energy dense foods (EDFs) and energy dense beverages (EDBs) as are typical of a “Western” dietary pattern, and physical inactivity among a sub-group of adolescents in Nairobi, Kenya. Also, the sources of food in the school environment, and the attitudes to the importance of diet and physical activity for health were assessed. A cross-sectional survey was conducted among 402 adolescents aged 13-19 years. The adolescents were recruited from schools that cater to students from households likely to be of at least middle-income socio-economic status. Results indicate that the majority of adolescents reported the consumption of an EDB (82.3%) and EDF (88.3%) at least once during the school day. Nearly half of the adolescents consumed a meal sourced from a school lunch program (45.9%). In terms of weekly consumption, the items that were reported as frequently consumed by the highest proportion of adolescents were sugar sweetened beverages (46.5%), and pastries such as biscuits and cakes (38.8%,). Both the occasional and frequent participation in team sports (50.5%) more common than individual (27.4%) and gym based workouts (26.4%). The majority (88.1%) of adolescents acknowledged the importance of diet and exercise for health. Rigorous measurement of diet and physical activity behaviour and knowledge and attitude of health behaviour in this thesis was limited. Also, the sample used was not representative of adolescents in Kenya. The findings of this thesis are preliminary and further research is recommended using a representative sample and validated data collection methods. Research of this nature can be used in the adoption of school food and exercise policies to promote healthy behaviours and, on a national level, the development of dietary guidelines for adolescents.