Wireless sensor networks (WSNs) consist of sensor nodes. These networks have huge application in habitat monitoring, disaster management, security and military, etc. Wireless sensor nodes are very small in size and have limited processing capability very low battery power. This restriction of low battery power makes the sensor network prone to failure. Data aggregation is very crucial technique in wireless sensor networks. With the help of data aggregation we reduce the energy consumption by eliminating redundancy. In this paper we discuss about data aggregation and its various energy-efficient technique used for data aggregation in WSN.