This volume includes 14 papers from the National Academy of Engineering’s Ninth Annual U.S. Frontiers of Engineering Symposium held in September 2003. The U.S. Frontiers meeting brings together 100 outstanding engineers (ages 30-45) to learn from their peers and discuss leading-edge technologies in a range of fields. The 2003 symposium covered these four areas: environmental engineering; fundamental limits of nanotechnology; counterterrorism technologies and infrastructure protection; and biomolecular computing. Papers in the book cover topics such as microbial mineral respiration; water-resource engineering, economics, and public policy; frontiers of silicon CMOS technology; molecular electronics; biological counterterrorism technologies; Internet security; DNA computing by self-assembly; and challenges in programming living cells, among others. A talk by Aerospace Corp. president and CEO William F. Ballhaus, Jr. titled The Most Important Lessons You Didn’t Learn in Engineering School is also included in the volume. Appendixes include summaries of the breakout session discussion that focused on public understanding of engineering, information about the contributors, the symposium program, and a list of the meeting participants. The book is the ninth in a series covering the topics of the U.S. Frontiers of Engineering meetings.