This comprehensive text shows the effectiveness of using mobile sinks in obtaining potential energy savings for the sensors in wireless sensor networks. The entire sensor network is divided into two layers: the resource-constrained sensor nodes constituting the lower layer and a mobile ad hoc network of resource-rich sinks forming the upper layer. Each sink is responsible for monitoring and data collection from a particular cluster of sensors. A sink moves to the vicinity of the sensor nodes - in a cluster - to collect data and exchanges the collected data with peer mobile sinks. The data can be transferred to the base station through multi-hop sink-to-sink data propagation and the energy loss incurred due to multi-hop data propagation in disseminating the data from a sensor to the control center is accounted to the sinks rather than sensors. This book also illustrates the scenarios, use just one mobile sink - where use of multiple mobile sinks becomes unaffordable - to reduce energy consumption in wireless sensor network. The lone mobile sink transfers the collected data to the base station directly.