Logistics activities are at the heart of world trade, but they also have unintended consequences on the environment due to the use of land, energy, and other types of natural resources. The significant energy usage by the more traditional means of transportation results in emissions, one of the most prominent of all negative externalities, that in turn causes air pollution affecting human health. One way to reduce such externalities is the (re-)design of the overall network on which logistics activities take place, giving rise to green network design problems, where the minimization of emissions is an integral and explicit part of the objective. The aim of this chapter is to present an overview and a classification of green network design problems arising at different levels of decision making, from operational to strategic, and will present definitions, optimization models, and practical applications for some of the key problems in this category.