To ensure widespread deployment and popularity, next generation wireless services will require a Quality of Service (QoS), and particularly a reliability, that is independent of the radio transmission medium. However, because of the failure-prone nature of wireless networks, providing a reliable communication link and guaranteeing a consistent QoS to users become key issues. In this tutorial, we describe the most common source of failures in wireless networks and provide a systematic failure classification procedure. Drawing from the vast literature on reliability in wireline networks, we then explain how cognitive radios can use their inherent capabilities to implement efficient prevention and recovery mechanisms to combat failures and thereby provide reliable communications and consistent QoS under all circumstances.