Wireless Sensor Network (WSN's) is an emerging technology that shows great promise for various futuristic. These application often used in many applications in military, ecological, and health-related areas. Therefore security in WSNs is very important. In this paper, the authors outline the security requirement and attacks with their corresponding counter measure in WSNs. Then present holistic view of security issues. These issues are classified into five categories cryptography, key management, secure routing, and data aggregation and intrusion detection.