Wireless Sensor Networks (WSNs) have become a prominent technology for many applications due to its various advantages and feasibility. It has been applied in government, military, transport, health care, education, business, and environment applications. WSN makes sensing, tracking, monitoring, and automation much simpler, effective, and efficient as compared to its predecessor technologies. Due to its vast implementations and immense potential, it has also attracted many threats and vulnerabilities along its path to advancement. This chapter reviews the existing literature and discusses the security issues related to WSN and the challenges in addressing them.