Wireless Sensor Networks (WSNs) are a special type of ad-hoc network. WSNs are easily subjected to intentional or unintentional attacks as compared to wired based networks. Denial-of-Service (DoS) attack is one of the main threats that the network is facing. DoS attack makes use of many hosts to send a lot of useless packets to the target in short time of invalid access which will consume the targets resources and causes outage of server operations. In this paper, the authors will discuss various types of attacks on WSNs and emphasize on DoS attack.