It's the most exclusive fraternity on Earth.
And it was 40 years ago today that the first of those men blasted off into space on the missions of their lifetimes.
Apollo 11 launched from Cape Kennedy on July 16, 1969, carrying Neil Armstrong, Michael Collins and Edwin "Buzz" Aldrin.
Four days later, the world watched as Armstrong stepped onto the moon and made his "giant leap for mankind."
Between 1969 and 1972, five more NASA missions landed on the moon, giving a total of 12 people the chance to walk, run and, even, golf on the lunar surface.
"For the astronauts, going to the moon was professional first and a personal experience second," said Andrew Chaikin, a science journalist who has spent more than 100 hours interviewing the lunar astronauts for his books "Voices From the Moon" and "Man on the Moon."
"Imagine getting to do the most important thing that you could possibly do in your profession, doing it for national prestige and doing it with the whole world watching and knowing that it was something only a handful of people could accomplish," he continued. "That's what it was."
These men were as prepared as they could be for their out of world experiences.
"But when they came back from the moon, we gave them the mission they never trained for," Chaikin said.
Fame, adulation, the constant stream of questions. These were the challenges they wrestled with in the aftermath.
"But they came through. And they came through in spectacular fashion," he said.
Some struggled more than others -- and some changed more than others – but they were all, in some way, moved by having seen Earth shrink before their eyes.
Here are their stories.
His were the first human feet to touch the extraterrestrial.
As the world watched on July 20, 1969, Neil Armstrong stepped onto the moon and gave American history one of its most memorable quotes.
But, though he's the most famous moon walker, he's also one of the most quiet, very selective about offers to speak and be interviewed.
He resigned from NASA in 1971 and accepted a teaching position at the University of Cincinnati's Department of Aerospace Engineering, which he held until 1979.
For the next decade or so, he was chairman of an aviation software company, Computing Technologies for Aviation, Inc. in Charlottesville, Va.
Along the way, he's collected honors from 17 countries, including the Presidential Medal of Freedom, the Explorers Club Medal and the Congressional Space Medal of Honor.
He had two sons with his first wife Janet, who divorced him in 1994. He later re-married.
In 2005, he stepped back into the public spotlight to endorse a biography, titled "First Man" by James Hansen. The book was as much an exploration of "American hero worship" as it was an exploration of the man himself.
"Friends and colleagues all of a sudden looked at us, treated us slightly differently than they had months or years before when we were working together. I never quite understood that," Armstrong said.
In an interview with CBS News' 60 Minutes soon after the book's release, he commented on his discomfort with his celebrity.
"I guess we all like to be recognized not for one piece of fireworks, but for the ledger of our daily work," Armstrong said. "I wasn't chosen to be first. I was just chosen to command that flight. Circumstance put me in that particular role. That wasn't planned by anyone."