Showcase #34-36 (October-December 1961): The introductory of Ray Palmer, the Silver Age Atom, and his girlfriend Jean Loring. These issues were the first adventures and the ones that helped him gain a more regular spot in the DC Universe due to the success of these stories.
The Atom #1 (June 1962): The start of his own solo comic series.
Justice League of America #14 (September 1962): The Atom joins the JLA as it's 9th member.
The Atom #7 (July 1963): The first pairing between the Atom and Hawkman.
Atom and Hawkman #39 (November 1968): Due to low sales of both their titles the Atom and Hawkman get their own title with the Atom's original numbering.