The social patterning of harmful health behaviours (HHBs) is a major determinant of the social gradient in non-communicable diseases, and health generally. For this reason, more research is needed to understand why HHBs vary according to the social and economic position (SEP) of individuals in this structured manner. As middle-childhood and adolescence is recognised as a `critical' period for the formation of HHBs the data used for this thesis cover the period from age 9 to age 20. Previous research has often adopted an individualist paradigm that focuses on providing `bottom-up' explanations for health inequalities i.e. how individual choices and decisions aggregate to produce inequalities. In comparison, a structural paradigm considers the role of `top-down' processes. According to this paradigm, HHBs are a response to conditions imposed by social structures and must be understood in their social and economic context. Drawing on longitudinal data from the Growing Up in Ireland (GUI) 98 Cohort, this thesis sets out to contribute to both perspectives.