In this chapter, we introduce fault tolerance in Wireless Sensor Networks. Firstly, we start with a short description of sensor networks, fault tolerance and its different techniques. Then we discuss the different phases of fault tolerance (fault models, fault detection and identification at five levels of abstractions (physical, hardware, middleware, system software and applications) and four scopes (components of individual nodes, individual node, network and the distributed system). The technical cores of the chapter covered various coverage strategies to make the system fault tolerant. We conclude the chapter with a brief survey of the future directions for fault tolerance in Wireless Sensor Networks.