We are pleased to announce the completion of this special issue. There were 27 submissions in total and 13 out of them were accepted after several rounds of review by the invited reviewers and the guest editors. The wireless sensor networks are emerging as a useful configuration of sensor entities that are capable of providing solutions for diverse applications under various unexpected and difficult situations. Recently, these networks are permeating in almost all areas where sensing of some

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... sical quantity is the primary application. In addition to sensing applications, these networks are used to further assist in evaluating different situations in tandem with numerous higher level applications. While the application of sensor networks seems to be diverse, there are many challenges that need to be mitigated in order to effectively implement practical systems. Due to the paucity of resources and unexpected nomenclature of these networks, it is difficult to control and perform management operations in an effort to satisfy the requirements imposed by various applications of the users. The operations of the wireless sensor networks are seriously hampered by various limitations pertaining to energy management, optimum bandwidth utilization, and connectivity. In order to cater to personal, commercial, and military applications, it is important to improve the sensor network lifetime. The distributed and volatile structure of the sensor nodes that are sometimes deployed in eccentric environments, where it is difficult to control these networks, may render access to sensors or reduce lifetime if the energy is not efficiently managed. In monitoring applications, which is typically critical mission, it is important to ensure efficient management, accuracy, and reliability for sensing applications and associated hardware. The accuracy also plays an important role where wireless sensor networks are deployed for tracking applications. Efficient algorithms are implemented that process critical data gathered from the deployed sensors in order to accurately achieve tracking capability. The cognitive radios are emerging and evolving in response to requirements of the current bandwidth hungry users, which require reliability and higher throughputs in already congested and scarce spectrum. Although the cognitive radios are evolving with their own issues and challenges, the amalgamation with sensor networks is also promising, as numerous requirements of the sensor network users may be accommodated. The cognitive radio network paradigm introduces competition and requires a high degree of etiquettes in accessing network for efficient access to resources. The application of game theory in a multidisciplinary approach provides an amicable solution to the spectrum access competition in addition to its suitability to several other problems. Some of the recent trends addressed by this special issue are energy efficiency, monitoring and tracking applications, cognitive radios, and the application of game theory to sensor networks. It is pivotal to incorporate a cross-disciplinary approach to resolve issues related to the fundamental problems in these networks. Energy Efficiency. The use of energy resources in an efficient manner influences the wireless sensor network lifetime. This efficiency in energy consumption can be achieved at different levels of sensor network designs. The incorporation of energy-efficient algorithm using the cross layer paradigm with alternate energy resources also improves the utilization, thus enabling multiple technologies with their inherent advantages and useful characteristics to be used in a single system. In order for the energy efficiency to be credible and useful for the entire wireless sensor networks, a mechanism must be defined to answer some of the following important questions. (i) Is the physical layer implemented while keeping in view the energy efficiency requirements and standards? (ii) Is the energy efficiency not compromised as a result of mitigating errors and the choice of routing methods? (iii) The wireless sensor network consists of distributed sensor nodes striving to achieve consolidated objective of providing sensor data to a destination. Are the sessions between several entities in the network complying with energy efficiency requirements? (iv) The applications also require being energy aware and must be capable of adjusting according to the requirements set forth by energy-efficient algorithms. Is the application layer in conformation with the energy efficiency requirements? (v) The cross layer approach is emerging as a valuable method to increase energy efficiency in addition to other vital parameters. Does the system incorporate energy efficiency by using cross layer approach? (vi) In order to achieve highly desired perpetual operation of the wireless sensor network, several renewable energy sources are utilized. Are we moving towards this method to increase the network lifetime? Monitoring and Tracking. An important application of wireless sensor networks is the monitoring of physical quantities used for a wide range of applications. The monitoring applications are useful in helping people with disabilities, efficiently managing sensed medical data and diverse industrial applications. The sensors are capable of providing sensed data, which is sometimes not possible by conventional acquisition methods. The tracking applications with the implementation and use of wireless sensor networks are also useful in wide range of applications. While these applications are very attractive in combating some difficult-to-address issues, there are still open questions that are important in this area of research.