The main goal of data-aggregation algorithms in wireless sensor networks (WSNs) is to gather and aggregate data in an energy efficient manner so that network lifetime is enhanced. In this article, we focus on QoS-based data-aggregation problems in WSNs. Firstly, we provide a definition of the different QoS parameters for data aggregation, such as energy efficiency, network lifetime, data latency and data quality. Then, we compare the different algorithms on each QoS parameter, describe the main features of each algorithm, and highlight the trade-offs between each parameter. Finally, we conclude with possible future research directions on QoS-based data aggregation in WSNs.