Wireless Sensor Network (WSN) is a collection of intelligent sensors that communicate and coordinate in an energy constrained environment. Due to the limited energy supply of the sensors, lifetime problems arise in static WSN. One of the potential solutions to this problem is a mobility-assisted WSN. Mobility can be introduced by adding extra entities like mobile sinks, mobile cluster-head or Mobile Relays (MRs) to the existing static WSN. The entities can significantly improve the functionality and performance of the WSN by making it flexible to failures, ease data collection, increase energy efficiency, enhance connectivity, improve coverage and prolong network lifetime.