It is hard to believe in security when it is so easy to access the communication media such as wireless radio media. However, the research community in industry and academia has for many years extended wired based security mechanisms or developed new security mechanisms and security protocols to sustain this marriage between wireless/mobile networks and security. Note that the mobile communication market is highly growing for different services and not only mobile phone service. That why securing wireless and mobile communications, is crucial for the perennial of the deployment of services over these networks.Wireless and mobile communication networks have gained a tremendous success in today's communication market both in general or professional usage. In fact, getting communication services anytime, anywhere and on the move has been an essential need expressed by connected people. This becomes true thanks to the evolution of communication technologies from wired to wireless and mobile technologies, but also the miniaturization of terminals. Offering services to users on the move has significantly improved productivity for professionals and flexibility for general users. However, we cannot ignore the existence of important inherent vulnerabilities of these unwired communication systems, which gives the network security discipline a key role in convincing users to trust the usage of these unwired communication systems supported by security mechanisms.Since the beginning of networking era, security was part of the network architectures and protocols design even if it is considered as slowing down the communication systems. Actually, network security is just a natural evolution of security of standalone or distributed operating systems dealing with machine/network's access control, authorization, and confidentiality and so on. Even though the context has changed from wired to wireless networks, we are facing the same issues and challenges regarding security. More precisely, it is about preserving the integrity, confidentiality and availability of resources and the network. Other security issues more related to the users such as privacy and anonymity are also important from the user's point of view today, especially with the new need of tracking criminals, but here in this book we are concerned only by the network security, and we also included two chapters dealing with important security issues and solutions to secure downloaded applications in the context of mobile operator, and copyright protection by watermarking techniques.Several security mechanisms have been developed such as authentication, encryption, access control, and others in order to offer secure communications over the network. According to the network environment, some security mechanisms are more mature than others due to the early stages of certain networking technologies such as wireless networks, ad hoc or sensors networks. However, even with maturity, and even if they are already widely implemented in marketed products some security mechanisms still need some improvement. It is also important to consider the limited resources of mobile terminals and radio resources to adapt wired network's security mechanisms to wireless context. These limited resources have direct impact on security design for this type of networks.MWNS 2009 is the second event trying to gather security and network experts together to present their research work in wireless and mobile networks security in different network architectures such as mobile, ad hoc and mesh, sensors and wireless networks.