Wireless sensor networks are very popular because of the reliable communication way solutions to military and commercial challenges. The low cost feature is the main advantage of wireless sensor network applications. A wireless sensor network is mostly used to monitor environmental and physical conditions. It includes temperature, sound, pressure etc. and used to link data through the network to a main location. Wireless sensor network plays an important role in the military as well as commercial applications. This paper is the review of various security threads and requirements in the wireless sensor network.