Cognitive radio is viewed as a novel approach for improving the utilization of a precious natural resource: the radio electromagnetic spectrum. The cognitive radio, built on a software-defined radio, is de- fined as an intelligent wireless communication system that is aware of its environment and uses the methodology of understanding-by-building to learn from the environment and adapt to statistical variations in the input stimuli, with two primary objectives in mind: • highly reliable communication whenever and wherever needed; • efficient utilization of the radio spectrum. Following the discussion of interference temperature as a new metric for the quantification and management of interference, the paper addresses three fundamental cognitive tasks. 1) Radio-scene analysis. 2) Channel-state estimation and predictive modeling. 3) Transmit-power control and dynamic spectrum management. This paper also discusses the emergent behavior of cognitive radio. Index Terms—Awareness, channel-state estimation and predictive modeling, cognition, competition and cooperation, emergent behavior, interference temperature, machine learning, radio-scene analysis, rate feedback, spectrum analysis, spectrum holes, spectrum management, stochastic games, transmit-power control, water filling.