Cognitive Radio (CR ) is an intelligent radio communication system, whose intelligence mostly comes from the Cognitive Engine (CE ). Based on the techniques of software-defined radio and with the support of machine reasoning and learning in artificial intelligence, cognitive engine implements the cognitive loop to realize the abilities of sensing, adaptation and learning in CR. Cognitive engine consists of the modeling system, knowledge base, reasoning engine, learning engine and interfaces. The key techniques are knowledge representation, machine reasoning and machine learning.