Wireless sensor networking is envisioned as an economically viable paradigm and a promising technology because of its ability to provide a variety of services, such as intrusion detection, weather monitoring, security, tactical surveillance and disaster management. The services provided by wireless sensor networks (WSNs) are based on collaboration among small energy constrained devices known as sensor nodes. The large deployment of WSNS and the need for energy efficient strategy necessitate efficient organization of the network topology for the purpose of balancing the load and prolonging the network lifetime. In this paper we review the previous work done in the field of crucial event monitoring in wireless sensor networks.