Software defined radio (or SDR) is becoming a popular method to develop wireless networking systems. This paper focuses on SDR for rapid prototyping of radio systems and for cognitive radio experiments. SDR is not restricted to experimental systems; from a commercial perspective, SDR shows cost-saving and potentially eco-friendly benefits; as in reprogramming instead of replacing base stations, and versatile remote maintenance that can reduce travelling for maintenance staff . Cognitive radio is becoming less a wish-list item and more a necessity to make better use of the electromagnetic spectrum and to accommodate larger number of network users. Most countries have broad swaths of the spectrum with specific license restrictions and dedicated for certain uses. A large part of this spectrum tends to go underutilized; but others, and especially free-for-all bands like the 2.4 GHz Wi-Fi band, become congested. Cognitive radio, building on the flexibility of SDR designs, has been shown to alleviate this problem. In this paper, the author elaborates with a brief SDR system scenario, which uses the Reconfigurable Hardware Interface for computing and radio (RHINO) platform developed at the University of Cape Town. The paper concludes by recapping the main points discussed and future expectations for SDR and cognitive radio.