A Cognitive Radio (CR) is a wireless communication device that adapts its transmission based on local spectrum usage information. It can assess available radio resources, spectrum rules, user needs, and more to ensure quality service. Originally conceived as an intelligent personal digital assistant, CRs aim to provide personalized services while efficiently using spectrum. Full Cognitive Radios, as envisioned by Mitola, may not be realized for another 20 years. Today, CRs focus on improving spectrum efficiency by identifying unused spectrum portions, minimizing interference, and obtaining spectrum usage data through spectrum sensing, databases, or pilot channels.