Five men in the history of the sport have accomplished the feat, eight have come up one major short, and then there's Phil Mickelson.
Jack Nicklaus, Tiger Woods, Gary Player, Gene Sarazen, and Ben Hogan -- that's the list of golfers Phil Mickelson is trying to join this year at the US Open.
That is golf's greatest list -- the guys who have won all four major championships in their career. This will be Lefty's first crack at it but let's look back first at where each of those five got the final piece of that immortal four-piece puzzle.
The craziest part about Hogan's slam is that he won two of the four after the car crash that almost killed him. The last was a four-stroke win at the 1953 British Open at Carnoustie.
That was the year the British Open conflicted with the PGA Championship, which kept Hogan from trying for the grand slam in a single year.
Hogan never played the British Open again
His final major was the one he's most famous for -- his "shot heard 'round the world" at the 1935 Masters. It was that major's second-ever playing and the first time Sarazen had ever played in it.
Player is the most diverse of this group. He won one of each of the four majors before winning his next five. He also did it at the age of 29 which is insane because that's how old Dustin Johnson and Webb Simpson are.
It came at the 1965 US Open at Bellerive Country Club in Missouri and it came in an 18-hole playoff which Player won by three over Ken Nagle after blowing a three-stroke lead with three holes left in the final round.
It was the only US Open that Player would ever win.
Nicklaus' career slam came even earlier. He did it at the 1966 British Open in a one-stroke victory over Doug Sanders and Dave Thomas at Muirfield.
The Big Cat did it at the age of 24 (!) and did it much like Player, one of each before he won his next 10. His career slam came at the British Open and the Old Course where he won by a laughable eight strokes over Thomas Bjorn and Ernie Els. He was the only golfer to shoot every round in the 60s that week.
He and Nicklaus are the only golfers with two of each major (and they both have three of each).
The reality is that Mickelson's career slam should have come last year when he won at Muirfield but because he blew the 2006 US Open, it didn't.
Will it come this year? Next year? Nobody knows but Phil thinks he's getting "at least one" in his career.
That would be good enough to join the greatest list in golf history.