This book explains Wireless Sensor Networks energy constraints of battery-powered sensor nodes necessitate energy-efficient communication. The major existing solutions are based on classical layered protocols approach, which leads to significant overhead. It is much more efficient to have a unified scheme which blends common protocol layer functionalities into a cross-layer module. In this paper, a efficient crosslayer protocol (XLP) is introduced, which achieves congestion control, data fragmentation, routing, and medium access control in a cross-layer fashion. Our approach combines medium access organization with routing. in a cross-layer simulation platform, the state-of-the- art layered and cross-layer protocols have been implemented along with XLP for performance evaluations. XLP significantly improves the communication performance and outperforms the traditional layered protocol architectures in terms of both network performance and implementation complexity.