The present book deals with how to use the precious wireless resources that are usually wasted by under-utilization of networks. We have been particularly interested to all resources that can be used in an opportunistic fashion using different technologies. We have designed new schemes for better and more efficient use of wireless systems by providing mathematical frameworks. In this sense we focused on cognitive communication networks, where a cellular service provider can reserve a portion of its resources to secondary users or virtual suppliers. Delay tolerant networks are used as an alternative to the significant increase of the traffic load in cellular networks. In areas where the network infrastructure implementation is geographically difficult, the use of ad-hoc networks seems an appropriate solution. Indeed, we have developed a new analytical model of IEEE 802.11e used in this type of network protocol.