The security of conventional or classical cryptography systems relies upon the supposed (but often unproven) difficulty of solving certain classes of mathematical problem. Quantum cryptography represents a new paradigm for secure communications systems since its security is based not on computational complexity, but instead on the laws of quantum physics, the same fundamental laws that govern the behaviour of the universe. For brevity, this paper concentrates solely on providing a simple overview of the practical security problems that quantum cryptography addresses and the basic concepts that underlie the technique. The accompanying talk will also cover this introductory material, but the main emphasis will be on practical applications of quantum cryptography in optical fiber systems. In particular, I will describe a number of experimental systems that have been developed and tested recently at BT Laboratories. The experimental results will be used to provides some insights about the likely performance parameters and application opportunities for this new technology.