In this article, we give an overview of current research on shared secret-key agreement between two parties. This agreement is based on radio wireless channel characteristics. We discuss the advantages of this approach over traditional cryptographic mechanisms and present the theoretical background of this approach. We then give a detailed description of the key-agreement process and the threat model, and we summarize the typical performance metrics for shared secret-key agreement. There are four processes in shared secret-key agreement: sampling, quantization, information reconciliation, and privacy amplification. We classify prior and current research in this area according to innovation on these four processes. We conclude with a discussion of existing challenges and directions for future work.