Wireless Sensor Networks (WSNs) consist of a set of small devices, called the sensor nodes, with sensing and wireless communication capabilities. Inspite of various applications, WSNs suffer from many constraints, including low computation capability, small memory, limited energy resources and susceptibility to physical capture. Also, sensor nodes are mostly deployed in potentially adverse or even in hostile environment. These constraints and issues make security in WSNs a challenge. So, efficient key distribution and management schemes are must. To address the tradeoff between above listed constraints and security, many key establishment techniques have been established. It is noticed that no key management technique is ideal to all scenarios where WSNs are used and the techniques employed should depend upon the requirements of the target applications and resources of each individual sensor network. In this book, a novel key establishment protocol has been proposed for sensor networks with different requirements. Clustering concept has also been implemented to improve the energy efficiency.