As the previous chapter showed, membership in International Organizations (IOs) is the only factor that seems to be robustly associated with environmental quality in developing countries. In Chapter 2, I argue that IO membership improves the environmental performance of developing countries because IOs can provide these countries with much needed resources in the form of knowledge and technology necessary to curb pollution. Furthermore, IOs can constitute a forum in which actors having an interest in environmental quality may successfully influence other countries’ behavior. Finally, in addition to increasing the reputational stakes for reneging on agreements, IOs also allow for issue linkage. This leads me to hypothesize that a developing country that is a member of more IOs provides higher levels of environmental quality. In order to better understand the mechanisms underlying this effect, this chapter evaluates the relationship between IO membership and environmental quality in greater detail. To this end, various strategies are pursued.