This chapter focuses on a cognitive wireless sensor network (WSN), where a primary wireless sensor network (PWSN) is co-located with a cognitive sensor network (CWSN). It provides a simple analytical model to characterize the local sensing performance per subchannel, in terms of probabilities of missed detection (MD) and false alarm (FA). The chapter proposes a joint source channel coding (JSCC) scheme, in the CWSN, to exploit the source correlation to improve the reliability of the final decision taken by the secondary fusion center (FC). It then analyzes the performance from a network point of view, distinguishing between uncoded and coded scenarios. The chapter also focuses on an uncoded scenario with error-free communication links. Dynamic spectrum access (DSA) has been considered to achieve more efficient radio spectrum utilization. To support DSA, secondary users (SUs) are required to capture or sense the radio environment, and an SU with such a capability is also called a cognitive radio (CR).