This book deals with risk assessment methodologies for network architecture design. The main goal is to present and to illustrate an innovative risk propagation-based quantitative assessment tool. This original approach aims to help network designers and security administrators to design and to build more robust and secure network topologies. As an implementation case study, we consider an aeronautical network based on the AeroMACS (Aeronautical Mobile Airport Communications System) technology. AeroMACS has been identified as the wireless access network for airport surface communications which will be soon deployed in European and American airports mainly for communications between aircraft and airlines. It is based on the IEEE 802.16-2009 standard also known as WiMAX. The first part of this book is an introduction to information system security risk management process. The second part presents the different risk management methodologies that can be used currently (quantitative and qualitative ones). In the third section, our original quantitative network risk assessment model based on risk propagation is introduced. Finally, the last section presents a network case study of the future airport AeroMACS system. This example illustrates how our quantitative risk assessment proposal can provide help to network security designers for the decision making process and how the security of the entire network may be hence improved.