How It's Done: That's not as important as the other question — how is it done without detection? It's an issue all sports are grappling with to some degree, and baseball's testing program has changed quite a bit since the 1990s.
Famous Examples: Mark McGwire, who hit 583 homers in his career, has admitted using steroids and human growth hormone. Rafael Palmeiro, who reached both the 500-homer and 3,000-hit marks, was suspended in 2005 for a positive test for PEDs. He claimed it was due to a vitamin vial given to him by a teammate.
Alex Rodriguez has admitted he used PEDs while with the Texas Rangers from 2001-03, before Major League Baseball and the players' union started their joint drug program.
Plenty of others are under suspicion, too — witness the Hall of Fame vote, in which baseball writers didn't give any player the 75 percent required for induction this year.