In this chapter, we outline the current state of research for the design of fault tolerant wireless sensor networks. First, we discuss the various notions of faults as concerns a wireless sensor network, including fault characteristics, sources of faults, and random faults versus faults with particular temporal, geographical, or topology-related patterns. Second, we discuss techniques for fault prevention and management including techniques for detection and recovery. Finally, we provide a particular focus on a fault management in cluster-based wireless sensor networks, and outline current research directions in architecture design and recovery techniques, followed by the chapter summary.