Data gathering in wireless sensor networks by employing mobile data collectors (MDCs) can greatly reduce the relay hops when the sensors transmit data to static base station. This prolongs the lifetime of whole network for the energy saving at sensors. To avoid the energy holes incurred by multi-hop relaying among sensors, MDCs gather data from the sensors directly or some nodes buffered data of other sensors. Furthermore, MDCs relay data from sensors to the base station when there are no complete links between them due the fact that some sensors were invalid. However, new challenges have arose for the mobility of MDCs in data gathering, which is different from static wireless sensor networks. This paper focuses on the mobility-assisted data of gathering strategies in wireless sensor networks. Some current novel theories and algorithms for data gathering based on MDCs are reviewed, and the taxonomy is described. More specifically, several typical algorithms and protocols are discussed in detail. In the end, advantages and disadvantages of the algorithms are summarized. The open research issues in this field are also pointed out.