The focus of our research in the area of network security is set on infrastructureless and decentralized communication networks. Particular technologies we are in touch with are mobile ad hoc, mesh, and sensor networks as well as peer-to-peer systems and service oriented architectures. We thus consider an infrastructureless and a decentralized nature on each layer of the network stack. The (multihop) wireless data transmission and the lack of central trusted instances render the resulting communication substrate a challenging environment with respect to security. Allowing for establishing enhanced communication services beyond the reach of infrastructure-based, centralized systems on the one hand, infrastructureless and decentralized systems are beyond the protection of 'traditional' security mechanisms on the other hand. Well-known and established security mechanisms such as gateways and firewalls to protect network borders or RADIUS and Kerberos to manage user-authentication and access control can not be applied directly in the infrastructureless and decentralized domain. To deal with these challenges, our research aims at identifying and classifying threats on infrastructureless and decentralized networks and, upon this basis, developing appropriate security mechanisms. Current research topics include