The cellular wireless market has begun the transition to data centric services including high speed internet access, video, high quality audio, and gaming. Communications technology can meet the need for very high data link speeds, and can also improve network throughput, but dramatically more spectrum will be needed to provide ubiquitous wireless data service. Cognitive radio is a new technology that allows spectrum to be dynamically shared between users. It offers the potential to dramatically change the way spectrum is used in systems and to substantially increase the amount of spectrum available for wireless communications. This paper introduces cognitive radio and explains the promise, possible operating modes, and benefits it may offer. Categories and Subject Descriptors A.0 [General]: Conference proceedings. General Terms Design. Keywords Cognitive radios, unlicensed spectrum, unlicensed wide area network.