Wireless systems have historically operated in relatively static frequency bands allocated to a single type of service.  This compartmentalization of spectrum offered few incentives for common designs across frequency bands and limited opportunities for dynamic spectrum sharing.  More recently, there has been a push to make more spectrum available, and in response there has been significant research towards developing new classes of “cognitive” radios and their associated protocols. Cognitive radios (CRs), or more generally software defined radios (SDRs), can tune their communication stack to take advantage of spectrum opportunities, adapt to a wide variety of radio interference conditions, and employ protocols that allow for collaboration between previously incompatible technologies. Dynamic physical layer adaptation is achieved by scanning available spectrum, selection from a wide range of operating frequencies, rapid adjustment of modulation waveforms and adaptive power control.  A suitably designed cognitive radio with a software-defined physical layer would be capable of collaborating with neighboring radios to ameliorate interference using higher-layer protocols.  These higher layer coordination protocols could range from etiquette mechanisms to fully collaborative multi-hop forwarding.  The potential impact of cognitive radios upon both commercial and military wireless communications is great as they could allow for improved spectrum utilization and improved interoperability between wireless data standards.