When a player reaches the 30 home run level and the 30 stolen base level in one given season, he is said to be a member of the 30-30 club.
Membership in this exclusive club is a good indicator of a player who combines some of the core skills of a baseball player: speed and hitting with power. Those two attributes are often at odds with each other: swift players usually do not have consistent home run power, and power hitters are not often known as prolific base stealers. Players in the 30-30 club can not only fly around the bases but also launch a home run.
The number of players in this club ballooned at the height of MLB's Steroid Era in the 1990s--when 20 such seasons were recorded--but it became a more restricted group shortly thereafter, when the club was cracked just 12 times between 2000 and 2010. Baseball's first 30-30 player was Ken Williams in 1922, with 39 home runs and 37 stolen bases. Bobby Bonds and his son Barry were well-known and frequent members of the club, each achieving five such outstanding seasons. Most members of the club have been outfielders; no pitchers or catchers have achieved such double excellence.