Wireless sensor networks are composed of sensor nodes which are limited in computational capabilities,power and memory resources,and they are susceptible to a variety of security threats.Security issues in WSNs have are the research focus in academy and industry.First,the early existing research results in wireless sensor network security was briefly surveyed,and then the security issues in WSNs were classified into five categories: cryptography algorithms and key management,secure routing,secure data aggregation,secure location,and privacy preservation.The mechanisms and characteristics of various techniques of these issues were described,while the focus was put on these new techniques that had been developed in recent three years.Finally on the basis of a comprehensive comparison and analysis of these existing techniques,it is points out that the heterogenous WSNs,inside attack and defensive techniques as well as privacy preservation techniques are the hot topics and future research trends in WSNs.