This book provides a short introduction to the basic theory behind the principle of marginal cost pricing, also discussing more practical and political considerations that, along with the theoretical models, form the ideal foundation for road user charges. The major focus is put on the marginal road wear. Different approaches to estimating road wear, and to give concrete examples of such studies are reviewed, in order to establish the background for the empirical study presented. A model is developed as a first effort to estimate marginal road wear on in-service roads in Norway. The data is partly based on Weigh-In-Motion data collected from the Automatic Traffic Control units on the Norwegian road network. This information about factual traffic loads is combined with measurements of the development of rutting and roughness recorded in the Norwegian Road Data Bank. Based on this a complete calculation model for external marginal costs is developed. This book is a slightly revised version of my PhD Thesis. No alterations have been made to the data presented or the analysis. The alterations made are only of a textual nature.