Time synchronization is a new focus topic in the research of Wireless Sensor Networks. Many applications of WSN need clocks of sensor nodes to be synchronized. However, Time Synchronization for WSN has unique requirements for scope, energy-consumption and precision of synchronization result from its own characteristics, making traditional synchronization methods unsuitable for WSN. The existing typical time synchronization algorithms for WSN are discussed and presented in detail, and the future directions of research on time synchronization for WSN are forecasted.