Among both academics and the wider development community there seems to be a general acceptance of the value of good governance and its role in promoting economic growth. However, beyond this general statement, there is a lack of deeper theoretical understanding as to why good governance is expected to foster economic growth and how such effects may take place. We de.ne governance quality as the capacity of a government to internalize externality. A theoretical model is developed to formally integrate governance quality into an endogenous growth framework. We elucidate the underlying mechanisms, through which governance quality affects economic performance: governance quality affects the productivity of public investment and in turn has an impact on economic performance. We also highlight that the endogeneity of governance quality and development stages have strong implications for the governance-growth relationship.