This project develops an 8-week middle-school mathematics module that introduces cryptography, the science of sending secret messages, while teaching and reinforcing the learning of related mathematical concepts. The topics range from the classical encryption systems and the historic context in which they were used through powerful modern encryption systems that provide secrecy in electronic messages today. The module also covers passwords and codes that correct errors in the transmission of information. Public awareness of the importance of cryptography is growing, as is the need to understand the issues involved. The study of cryptography provides an interesting context for students to apply traditional mathematical skills and concepts. Mathematical topics covered include percents, probability, functions, prime numbers, decimals, inverses and modular arithmetic. The main product is a middle-school student book, with accompanying teacher materials. A web site is being developed that supports the activities in this book. Abbreviated modules for Grades 3, 4 and 5 are also being developed, as well as an instructor's guide for adapting the materials for use in informal educational settings such as museums and after-school programs. The development of the module involves piloting and field-testing by experienced classroom teachers from diverse school communities and instructors of informal educational programs. Evaluation includes review by mathematicians and educators, as well as an investigation into the level of students' understanding of the topics studied.