The planet Mars has long held a special fascination and even mythic status for humans. While not the closest planet to Earth, scientists have considered it to be the planet that most closely resembles Earth and thus is the other planet in our solar system most likely to contain life. Since before the space age began, people have wondered about the "red planet" and dreamed of exploring it. In the twentieth century, robotic spacecraft and then human space flight became a reality. Those who wanted to explore Mars in person felt that this might finally become a reality as well. The Apollo program, which put twelve Americans on the surface of the Moon, certainly encouraged the dreamers and planners who wanted to send astronauts to Mars. Indeed, many people in and out of the National Aeronautics and Space Administration (NASA) have felt that human exploration of Mars is the next logical step in human space flight after the Moon. Clearly, however, many obstacles have remained. Human travel to and from Mars probably would take many months at best. Thus the biomedical and psychological implications of such long-duration missions are daunting. The logistics of getting enough food, water, and other supplies to Mars are also challenging at best. What would astronauts do once they got to Mars? How long would they stay on the planet's surface and how would they survive there before returning to Earth? The financial cost of sending humans to Mars would almost surely be measured in billions of dollars. Aside from technical and financial issues, there remains the political question of why we should send humans to Mars at all. David Portree takes on these questions in this monograph. By examining the evolution of 50 mission studies over the past 50 years, he gives us a sense of the many options that Mars human space flight planners in the United States have explored. Portree covers a wide variety of ideas for human exploration of Mars, ranging from Wernher von Braun's of the 1950s to the Space Exploration Initiative of 1989. These concepts, culled from a much larger pool of studies, range from hugely ambitious flotilla-style expeditions to much leaner plans. This monograph provides historians, space policy practitioners, and other readers with a very valuable overview of how much planning has already been done. If humans do go to Mars any time in the near future, it is quite conceivable that their mission profile will resemble one of the plans described here. NASA SP-2001-4521