"Objectives: To examine the prevalence of overweight and obesity in Australian secondary school students and identify factors associated with excess adiposity. Design, setting and participants: Cross-sectional survey of students aged 12-17 years (in school years 8-11) who completed the National Secondary Students' Diet and Activity survey in 2009-10, which included a web-based self-report questionnaire and height and weight measurements. Main outcome measures: Overweight and obesity based on international standard body mass index (BMI) cut-offs for children and adolescents. Results: Data were analysed for 12188 students. Just under one in four students were either overweight (18%) or obese (5%). After adjusting for demographic and health-behaviour characteristics, males were more likely than females to be overweight or obese (OR, 1.23; 95% Cl, 1.07-1.40; P = 0.004), as were both low (OR, 1.67; 95% CI,1.40-1.99; P < 0.001) and medium (OR, 1.33; 95% CI, 1.14-1.55; P < 0.001) socioeconomic position (SEP) students compared with high SEP students. Students engaging in low levels of physical activity (OR, 1.21; 95% Cl. 1.08-1.36; P = 0.001), more time in small-screen recreation (OR, 1.18; 95% Cl, 1.05-1.32; P = 0.005), and short sleep duration (OR, 1.22; 95% CI, 1.05-1.41; P = 0.008) also had higher odds of being overweight or obese. Conclusions: There is a need for interventions to reduce overweight and obesity during adolescence. Preventive measures should include a focus on facilitating physical activity and reducing sedentary behaviour, as well as promoting adequate sleep, particularly among young people from lower SEP neighbourhoods who appear to be most susceptible."