Quantum cryptography is a rapidly developing field that draws from a number of disciplines, from quantum optics to information theory to electrical engineering. By combining some fundamental quantum mechanical principles of single photons with various aspects of information theory, quantum cryptography represents a fundamental shift in the basis for security from numerical complexity to the fundamental physical nature of the communications channel. As such, it promises the holy grail of data security: theoretically unbreakable encryption. Of course, implementing quantum cryptography in real broadband communications systems poses some unique challenges, including generating single photons, distilling random keys from the quantum key distribution process, and maintaining security at both the theoretical and practical level. Overall, quantum cryptography has a place in the history of secret keeping as a novel and potentially useful paradigm shift in the approach to broadband data encryption.Table of Contents: Introduction / Elements of Classical Cryptography / The Quantum Mechanics of Photons / Fundamentals of Quantum Key Distribution / Information Theory and Key Reconciliation / Components for Broadband QKD / A Survey of QKD Implementations / Conclusion - QKD in the Marketplace