Cognitive Radio (CR) is a revolutionary technology aimed at optimizing the utilization of the electromagnetic spectrum, a limited and valuable resource. Despite its promising potential, the deployment and widespread adoption of CR face several technical, regulatory, and practical challenges. This paper presents a comprehensive study of the key research challenges in Cognitive Radio Networks (CRNs). We begin with a chronological literature survey, highlighting significant advancements and ongoing research efforts. Subsequently, we delve into the current research challenges, including spectrum sensing, dynamic spectrum access, security, energy efficiency, interoperability, and regulatory issues. We also explore potential research opportunities that could address these challenges, thereby paving the way for more robust and efficient CRNs. This review aims to serve as a foundational reference for researchers and practitioners in the field, offering insights into future research directions.