By examining the evolution of 50 mission studies over the past 50 years, David S. F. Portree 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.