Scott Lang was an electronic engineer who was unable to support his family, and decided to turn to crime. He was caught and sentenced to 5 years in jail, but got out early for good behavior. Once freed he began to work at Stark International's design department.
When his daughter was diagnosed with a fatal heart condition, Lang sought help from Dr. Erica Sondheim, who was kidnapped by Cross Technological Enterprises founder Darren Cross, for help with his own heart condition. In order to rescue her, and save his daughter, Lang broke in to the home of Dr. Henry Pym, the first Ant-Man, and stole his technology. After a successful rescue, Lang tried to turn himself in, but Pym allowed him to keep the suit, but for the purpose of good.
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Ant-Man (Scott Lang) - Marvel Universe Wiki: The definitive online source for Marvel super hero bios.