In Major League Baseball (MLB), the 30–30 club is the group of batters who have collected 30 home runs and 30 stolen bases in a single season. Ken Williams was the first to achieve this, doing so in 1922. He remained the sole member of the club for 34 years until Willie Mays achieved consecutive 30–30 seasons in 1956 and 1957. Bobby Bonds became the club's third member in 1969 and became the first player in MLB history to reach the 30–30 club on five occasions, subsequently achieving the milestone in 1973, 1975, 1977 and 1978. He remained the only player to accomplish this until 1997, when his son Barry Bonds achieved his fifth 30–30 season.
In total, 38 players have reached the 30–30 club in MLB history and 13 have done so more than once. Of these 38 players, 26 were right-handed batters, 8 were left-handed and 4 were switch hitters, meaning they could bat from either side of the plate. Ten of these players (including eight active members of the 30–30 club) have played for only one major league team. The Cincinnati Reds, Colorado Rockies and New York Mets are the only franchises to have three different players reach the milestone while on their roster. Five players—Hank Aaron, Barry Bonds, Willie Mays, Alex Rodriguez and Sammy Sosa—are also members of the 500 home run club, and Aaron, Mays and Rodriguez are also members of the 3,000 hit club. Dale Murphy, Jose Canseco, Barry Bonds, Larry Walker, Jimmy Rollins and Braun won the Most Valuable Player (MVP) Award in the same year as their 30–30 season, with Bonds achieving this on two occasions (1990 and 1992). Both Mays and Rollins also reached the 20–20–20 club in the same season, that being Mays second 30-30 season, and in Rollins only 30-30 season. Four different players accomplished 30–30 seasons in 1987, 1996, 1997 and 2011, the most in a single season.
Due to the rarity of a player excelling in the combination of hitting home runs and stealing bases, Baseball Digest called the 30–30 club "the most celebrated feat that can be achieved by a player who has both power and speed." Of the 22 members eligible for the Baseball Hall of Fame, four have been elected and two were elected on the first ballot. Eligibility requires that a player has "been retired five seasons" or deceased for at least six months, disqualifying fourteen active players and two players who have been retired for less than five seasons.