Key management in Wireless Sensor Networks (WSNs) is an important issue due to the absence of trusted infrastructures, on one hand, and the limited resources of sensor nodes, on the other hand. This paper surveys some recent key management approaches in WSNs. It first identifies some of the problems that confront the key management. Then, it defines some criteria for viable solutions to key management problems. Next, it explores some of the proposed key management approaches, and analyzes them according to the presented criteria. Some open research issues are discussed.