Wireless sensor networks (WSN) are mobile ad hoc networks in which sensors have limited resources and communication capabilities. Secure communications in some wireless sensor networks are critical. Key management is the fundamental security mechanism in wireless sensor network. Many key management schemes have been developed in recent years. In this paper, we present wireless sensor network key management survey and taxonomy. We classify proposed wireless sensor network key management schemes into three categories based on the encryption key mechanism. We then divide each category into several subcategories based on key pre-distribution and key establishment.