The thesis presents a secure communication architecture for net centric operations which is a relatively new paradigm for gaining superiority in the battlefield. The net centric operation (NCO) dictates communication as an enterprise to maximize performance for reducing the cost by optimally allocating resources and functionality across terrestrial, air and space layers. The net centric operation connects sensors, communications systems and weapons’ systems on information grid for providing real time information to war fighters, policy makers and support personnel. The proposed network architecture comprises of static nodes, semi-mobile nodes and fully mobile nodes. The static nodes form strategic network, fully mobile nodes form tactical network while semi-mobile nodes act as a gateway between strategic network and tactical network for long range connectivity. A secure cognitive network device (SCND) is presented to cater for the ubiquitous connectivity required for the proposed architecture. A cognitive layer is embedded in the SCND that integrates different networking and physical interfacing technologies together thus adding another dimension in the field of networking. The presented cross-layer architecture integrates different networking technologies by exploiting vertical handoffs between networks employing different standards. The cognitive layer is augmented by GPS device to learn the environment by remembering the locations of patches of areas where connectivity tends to drop. The cognitive controller routes the call on alternate carrier, having the best QoS in that patch. The cognitive device therefore improves the link reliability and network coverage by situational awareness and intelligent processing. The concept of SCND is further extended for secure mobile backbone architecture for tactical communication. The architecture comprises of mobile backbone nodes (MBNs) and simple mobile nodes. This architecture supports long range communication especially for mission critical applications like net centric warfare demanding rapid deployment of communication infrastructure. The MBNs form clusters and adjust their positions to provide optimum connectivity to their respective mobile nodes. The MBNs are equipped with SCNDs and serve as the cluster head or centroid of the cluster. When the mobile nodes in a cluster move, the MBN adjusts its position in such a way that on ivone hand it provides optimum connectivity to its cluster nodes and on the other hand, it remains connected to its backbone nodes. The MBN is assumed to have digital elevation information. This is accomplished in reality by augmenting the nodes with digital elevation models (DEMs). The height information helps for adjustment of different antennae positions to achieve line of sight according to the terrain. The proposed architecture also supports distributed computing with a novel framework of application migration. The mobile nodes with limited battery life and computational resources can offload their computational intensive applications to their corresponding MBNs. In the proposed architecture, as the mobile node moves to become a part of another MBN, its offloaded applications are automatically migrated to the new MBN. A specially designed programmable and scalable security processor is another significant component of secure network architecture. The programmability of the security processor enables the porting of current and proprietary security algorithms as well. The scalable and layered architecture supports very high data rates by instantiating multiple layers in the design. The modularity of the design makes it suitable for national secure mobile network infrastructure. The proposed SCND employed in tactical communication for NCO is also applicable in vehicular networks (VNet). Therefore, based on SCND, a secure cognitive vehicular communication architecture is also presented for dispatching location aware, safety critical and value added services in a unified vehicular network. The network incorporates communication nodes with hybrid network access technologies in vehicles and roadside infrastructure. The vehicular network consists of vehicle-to-vehicle, vehicle to roadside infrastructure and backbone communication with control and monitoring centers. The system incorporates integrated multi telecommunication technologies like HF, VHF, UHF, GSM, Satellite, Broadband Wireless and wired links. The architecture divides the entire geographical region into major and minor zones. Each major zone has its registrar to register and authenticate the nodes of its geographical area. The nodes in the minor zones form local MANETS to exchange safety critical messages and form heterogeneous networks for value added services.