Wireless sensor networks consist of a large number of small battery- powered devices which can be deployed to sense, gather process and transmit data from the environment to serve a numerous applications such as: emergency applications, health monitoring, scientific investigation, ambient intelligent, traffic control, surveillance systems and military applications. Most of these applications rely in extracting useful data from the environment and respond to user's queries or in some situations sensors take an active role to transmit the data to consumers. Sensors are constrained recourses due to the power and memory limitation which is the main issues in WSNs. Due to these limitations, the main goal in WSNs is maximizing the life time of the sensor nodes which lead to increase the network or the system lifetime. This goal can be accomplish by using an energy-efficiency algorithms to gather and aggregate data in WSNs. Data aggregation is the process of aggregating data that collected from different sensors to send this aggregated information to the base station. The purpose of data gathering and aggregation is to avoid transmitting redundant data and save energy. In this presentation, we will give an overview of different data aggregation algorithms in WSNs.