Although not the original hero named Atom (the first, Al Pratt being a tough-guy type), Ray Palmer, was the first heroic Atom that could change size. As the Atom of the Silver Age, Palmer made his comic debut in 1961's Showcase #34 clad in red and blue.
Dubbed the Tiny Titan, the Atom made his onscreen debut as one of the heroes featured in Filmation's DC Super Heroes series. The Atom starred in three short episodes as well as appearing in the three Justice League of America shorts.
Although not a mainstay, the Atom made several appearances as a guest hero during the multiple incarnations of Super Friends in the 70's and 80's.
It was during the 1979 Legends of the Superheroes variety show that the Atom made his live action debut. During the show, the Atom was "comically" presented as romantically engaged with the villainous Giganta.
Following the end of the Super Friends, the Atom would take a bit of a breather before attempting a comeback in the failed 1997 Justice League of America Pilot. Still wearing red and blue, this take on the Atom looked about as ridiculous as the rest of his teammates.
In 2000, The Call, a two-part episode of Batman Beyond was used as a test run for a Justice League series. The episode featured Justice League Unlimited, featuring heroes of the future. Although Superman was a member of the future League, other traditional members were replaced by similar archetypes - Big Barda filled in for Wonder Woman joined by Green Lantern Kai-Ro, Aquagirl, Warhawk, and Micron. Although Micron could shrink like the Atom, he could also grow bigger like Marvel's Ant / Giant Man.
2002's Justice League episode Legends featured a character known as Tom Turbine who was a partial nod to the original Atom who was a member of the Justice Society.
In 2004, the Atom returned from obscurity in Justice League Unlimited. The episode Dark Heart saw Ray hitching a ride with Wonder Woman's wonder twins.
Batman: The Brave and the Bold introduced the Ryan Choi Atom for the first time on screen in 2009. In Journey to the Center of the Bat! Atom and Aquaman teamed up with platelet as they traversed Batman's circulatory system.
During the first season Batman: The Brave and the Bold episode Deep Cover for Batman! Bats found himself at odds with evil counterparts to Earth's heroes including the menacing Dyna-Mite.
The original Atom, Al Pratt would make a couple of "tiny" cameos in the Absolute Justice Smallville event in 2010 including in painted form within a mural of the JSA. Al was seated third from the left. Despite his small size, the Atom had made a big impact on the DC universe, and its only a matter of time until he pops up again.