The number of security breaches is on a sharp increase and so is are the damage and losses. Although the actual amount of damage from malicious codes has not been fully revealed, it is enormous, and such damage occurs from common services such as in cases of game hacking, messenger phishing, voice phishing, and so on. Wireless sensor networks (WSNs) have recently attracted a lot of interest in the research community due their wide range of applications. Due to distributed nature of these networks and their deployment in remote areas, these networks are vulnerable to numerous security threats that can adversely affect their proper functioning. This problem is more critical if the network is deployed for some mission-critical applications such as in a tactical battlefield. Random failure of nodes is also very likely in real-life deployment scenarios. Due to resource constraints in the sensor nodes, traditional security mechanisms with large overhead of computation and communication are infeasible in WSNs. Security in sensor networks is, therefore, a particularly challenging task. Wireless Sensor Network (WSN) has wide area of application for real time event detection. The sensing capability of a WSN requires sensor nodes as a part of it. The sensor nodes are with limited resources and power. This makes a WSN vulnerable to many kinds of attacks. Denial of Service (DoS) attack is one of them. Each layer has different type of DoS attack. Tackling this attack requires knowledge of types of DoS as well as various defense mechanisms applied to overcome them. In this project, an introduction to DoS attack along with various countermeasures has been discussed.