Wireless sensor networks (WSNs) offer a wide range of applications due to their easy deployment, scalability, and auto configuration of nodes. In fact, the applications are so diverse that they have permeated in every aspect of human life and all the global applications. Today WSNs are a part of almost every application area. For the purpose of understanding varied applications, we need to segregate and classify them on the basis of similar characteristics, which is again dependent on 242the type of WSN. Sensors can be classified on the basis of behavior and deployment environment. Sensor nodes sense the environmental parameters and communicate to gateway via sink. The sensing pattern of the sensor nodes may be periodical or nonperiodical. Periodical sensing behavior is also called time-dependent sensing. Nonperiodical sensing is not time based but is dependent on some criteria such as event occurrence, etc. Different deployment environments have their specific architectures. Hence, the applications can be classified on the basis of environment in which the sensor is deployed. For example, a terrestrial environment may differ from an aquatic environment and hence the architectures of both of them are different. They have specific types of sensor nodes, communication media, and specific sets of protocols to work in accordance with the system to overcome the limitations of that environment. Hence, the applications can also be classified on the basis of deployment environment.