Lynn M. Fahey Health behaviors are an important contributing aspect of physical health and well-being, yet the structural conditions that may shape health behaviors differ across socio-demographic groups, including between men and women and across SES groups. Prior work on the gender gap in health behaviors has several limitations, including a focus on gender disparities in adult circumstances only; failure to account fully for gender gaps in participation in a variety of behaviors, including smoking, drinking, and weight status; a focus on outcomes in mid or late life, with less attention given to how participation in health behaviors emerges and unfolds across earlier stages of the adult life course; and a reliance on retrospective, self-reported measures of early youth that are somewhat limited in scope. This dissertation responds to these limitations by using a life-course epidemiological framework and employs longitudinal data from across the early life course to explore how gender conditions the relationship between early life circumstances and health behaviors– specifically alcohol use, tobacco use, and weight status using a nationally representative longitudinal dataset: The National Longitudinal Study of Adolescent to Adult Health (Add Health). Generally, results show that the features of early life which are predictive of health behaviors differ based on the particular outcome examined, and that gender does interact with early life circumstances to produce health behaviors. For example, in terms of smoking behavior, the results support that women who had access to cigarettes or had peer smokers during youth are at a lower risk of being a current smoker than their male counterparts with similar youth exposures. Additionally, with respect to drinking behavior, the results of this study suggest that gender moderates the relationship between youth circumstances and heavy episodic drinking only at the earlier time points in young adulthood. The results for weight status transitions in this study do not suggest that gender operates as a moderator in the relationship between youth circumstances and adulthood weight status transitions. Taken together, this body of work extends and provides links between the prior literature on early life circumstances, gender differences in health across the life course, and gender differences in health behaviors.