This book is designed for researchers specializing in cognitive radio communications, with a particular focus on signal processing perspectives. It delves deeply into the security aspects of cooperative spectrum sensing, exploring attack motives, mechanisms, and state-of-the-art defense strategies. Additionally, the book provides a comprehensive analysis of using deep-Q-network (DQN) learning to counteract security threats in cognitive radios. The frameworks developed for DQN learning applications are versatile and can be adapted for other learning frameworks. Readers are expected to have a background in stochastic processes, wireless communications, linear algebra, detection theory, calculus, and basic signal processing.