Future wireless networks are expected to be spectrum agile and use cognitive radio technologies. The spectrum regulatory committees have been taking measurable steps with regards to laying down the rules and implementing them, which will open the doors to dynamic spectrum network technologies based on cognitive radio. The usage of free spectrum sharing between primary users (licensed) and secondary users, being license-exempted and has been demonstrated in the recent experimental trails based on TV whitespace networks. The technique of dynamically accessing unused spectrum using Dynamic Spectrum Access (DSA) is gaining momentum. Several experimental trails on dynamic spectrum networks is being carried out in the TV-band spectrum to test the technology and enact regulations. In this paper, an overview of cognitive radio i.e. spectrum sensing, spectrum sharing, spectrum management and spectrum mobility are discussed. This paper will explore the dynamic spectrum technologies as a precursor to full-fledged cognitive radio networks.