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Wireless sensor networks (WSNs) have recently emerged as a new vital area in networking research. WSNs are deployed with large number of small sensor devices which are highly distributed and are able to organize flexibly into a network. Sensor nodes are able to sense physical data and transmit the gathered information to one or more base stations. When large number of nodes starts transmitting their data at the same time, congestion occurs at the nodes near to base station due to many-to-one nature of communication. Congestion in WSNs may lead to poor energy efficiency, higher packet loss, lower throughput and increased packet delay. Hence, congestion control is the major issue that should be considered while designing any transport protocol for WSNs. In this paper, we have discussed recently developed and proposed congestion control techniques for WSNs. Firstly, we emphasized on some unique characteristics of WSNs, then we discuss some guidelines for designing an effective congestion control scheme and then we have compared various congestion control techniques based on QoS parameters such as energy efficiency and fairness. Finally, we conclude our study by defining some future directions.