A tall, good-looking guy who Penny dates on and off, mostly because he's cute. He's not particularly bright, although he likes to think he knows something about science, to the amusement of Sheldon, Leonard, Raj, and Howard.
Zack's father owns a company which makes menus, which is how Penny met him. She dated him immediately after breaking up with Leonard (0323).
Some months later, Penny was in a restaurant with Bernadette and Amy when Zack walked in. Amy found herself sexually attracted to Zack, much to her dismay (0410).
Around the holidays in 2010, Penny started seeing Zack again. She admitted to Leonard that she only went out with him so she wouldn't be alone on New Year's Eve. The guys tried to make friends with him and actually invited him along to the comic book store and then asked him to dress up as Superman and go to a New Year's Even party with them as part of the Justice League (0411).
Zack is played by Brian Thomas Smith.