Critical discourse analysis was used to explore and discuss data on young people's knowledge and understanding of health, fitness and physical activity, selected from a wider study which focused on the role of secondary schools in effectively promoting physical activity. A mixed methods approach was utilised, involving an online survey to teachers in all state secondary schools in the UK (n=603 responding schools) and case studies centred on eight randomly selected state secondary schools from nine Government regions across England. Within each case study school, teacher interviews and pupil focus groups were conducted involving 17 teachers and 132 children aged 12-15 years, respectively. The healthism discourse was evident in the way young people talked about health, fitness and physical activity and two key themes emerged, these being: (i) issues with young people's knowledge and understanding of health, fitness and physical activity in the form of reductive, limited and limiting conceptions; conceptual confusion; a preoccupation with appearance, weight, fat, shape and size; limited progression in learning; and complexities in understandings; and (ii) divides between young people's health knowledge and health behaviour, and dilemmas underpinning these divides. Improved understanding of issues with young people's knowledge and understanding of health, fitness and physical activity and of divides and dilemmas regarding associated behaviours should assist in developing critical pedagogies which challenge the dominance and stability of the healthism discourse and more effectively promote healthy, active lifestyles amongst young people.