There is a substantial gap in the literature about the effect of mental health on physical health. In two empirical chapters, this dissertation aims to understand how mental health influent physical health using a dynamic panel model and explore alternative explanations for this relationship using mediation analysis. In the first empirical chapter I investigate the relationship between physical health and mental health while accounting for the dynamics of physical health. I find that better past mental health positively influences present physical health. Better past physical health is also shown to have a positive impact on present physical health. Estimation by subgroups shows heterogeneous effects across age groups but no differences between males and females. In the second empirical chapter I propose and test a mediation framework for the analysis of the causal pathways through which mental health and overall life satisfaction on physical health. I find the significant mediator is an only physical activity and show that the indirect effect of past overall life satisfaction and mental health on physical health are strongly mediated by physical activity.My results suggest that both past mental health and past physical health can be powerful tools in promoting better present physical health. Mediation analysis shows that well-being provides influence both directly to physical health and indirectly via physical activity to better physical health.