This chapter is the extended work of the paper titled “Genetic Algorithm for Data Aggregation Trees in Wireless Sensor Networks”, appeared in Proceedings of the Third International Conference on Intelligent Environments (IE), Ulm, Germany, September 24–25, 2007. presents a genetic algorithm (GA) to generate balanced and energy-efficient data aggregation spanning trees for wireless sensor networks. In a data gathering round, a single best tree consumes lowest energy from all nodes but assigns more load to some sensors. As a result, the energy resources of heavily loaded nodes will be depleted earlier than others. Therefore, a collection of trees need to be used that balance load among nodes and consume less energy. The proposed GA takes these two issues in generating aggregation trees. The GA is simulated in an open-source simulator, J-sim. The simulation results show that proposed GA outperforms a few other data aggregation tree-based approaches in terms of extending network lifetime.