This book is a compilation of scientific papers presented at a July 1991 conference which was scheduled to coincide with the 15th anniversary of the unmanned Viking landing on the planet Mars. The conference was planned to cover past, present and future missions to Mars, with the papers of past missions serving as an historic scientific base, and papers of the future missions to Mars serving as the main focus of the conference. Chapters are grouped into six sections: overviews, prior missions, rationale and benefits of future missions, robotic missions, systems concepts and operations, and technology for future missions.