The Mercury Seven astronauts were selected by NASA based on their performance on a series of extremely difficult tests.  After their selection, the astronauts started their training.  Their training consisted of numerous mental and physical tests inorder to make certain they knew the intrique details of the mission and mechinary, and to simultate the effects of space flight on their bodies.  Once this was completed the fun began.
    All of the Mercury Seven astronauts flew in successful Project Mercury missions, with the exception of Donald "Deke" Slayton due to a medical problem.  Each astronaut ventured in uncharted territory and exhibited extrodinary bravery.  After the completion of Project Mercury many of the astronauts continued at NASA, participating in flights in later projects, Gemini and Apollo.  These seven men laid the groundwork for all space exploration.