Increasing the level o f participation in habitual physical activity o f at least moderate intensity conducted by youth is a health promotion and a disease-prevention strategy. The aim o f this thesis was to investigate the benefits and determinants of active commuting to school among Irish adolescents. In a behaviour-specific and context-specific analysis, the perceived physical environment was examined as a determinant o f active commuting to school, and the potential confounding factors of distance and density were explored. Existing theoretical models describing the physical environment were evaluated and improved.