This thesis investigates the epidemiology of lifestyle behaviours and training injuries among British Army infantry recruits. In Study 1, the Military Pre-training Questionnaire (MPQ) was developed to assess multiple risk factors for injury (smoking; alcohol consumption; physical activity; diet; and injury history). Its feasibility and test-retest reliability was demonstrated in a representative sample (n = 58) with no evidence of systematic bias between administrations. The MPQ was subsequently completed by a large cohort of infantry recruits before commencing initial training (n = 1,960). This enabled its validity to be assessed in Study 2, which revealed that a simple, single-item measure of pre-training physical activity was a strong predictor of initial physical fitness levels and likely training outcomes. In Study 3, cross-sectional analyses of MPQ responses enabled the prevalence, co-occurrence and clustering of unhealthy lifestyle behaviours to be assessed. Although physically active, the majority of recruits entering infantry training smoked, drank hazardously and had low fruit and vegetable consumption. Six percent of recruits reported no unhealthy lifestyle behaviours, 20% reported one, 35% reported two, 31% reported three and 8% reported all four. Paired combinations of unhealthy behaviours were strongly associated, particularly smoking/drinking, smoking/inactivity and diet/inactivity. Finally, in Study 4, a high incidence of overuse injuries in infantry recruits was observed using a prospective study design. Recruits with lower pre-training fitness levels, low body mass and past injuries were exposed to a higher risk of sustaining a training injury, while there was no evidence to suggest that engaging in unhealthy lifestyle behaviours increased injury risk. Collectively, the four studies have enabled, for the first time, a comprehensive analysis of lifestyle behaviours and training injuries among British Army infantry recruits. The findings have important implications for military health improvement and injury prevention strategies.