Scott Lang stole an Ant-Man suit and "shrinking gas" canisters from Dr. Henry Pym, the original Ant-Man, in order to save the life of his daughter by breaking into a factory where a heart specialist was being held. Pym gave Lang permission to keep the uniform and a supply of gas after Lang had proven himself willing to use his powers on the right side of the law.
Scott Lang was an electronics expert who could not support his family doing repair work, so he turned his talents to burglary. Apprehended, he served his prison sentence and was paroled for good behavior. During his time in prison, however, his wife divorced him. Lang had furthered his studies of electronics while in prison and was soon hired by Stark International to work in its design department.
When his daughter Cassie was diagnosed as having a serious congenital heart condition, Lang sought the aid of surgeon Dr. Erica Sondheim, whom he believed to have been held prisoner at Cross Technological Enterprises (CTE). Desperate to help his daughter, Lang decided to resort to burglary. Breaking into the New Jersey home of Dr. Henry Pym, Lang stole his Ant-Man uniform and shrinking-gas canisters. Unknown to him, Pym observed the theft and in his guise as Yellowjacket, followed Lang, curious to see what use he would put the paraphernalia.
Garbed as Ant-Man, Lang broke into Cross and discovered Dr. Sondheim was held prisoner by Darren Cross, (later, the super-villain Crossfire) president of CTE, who needed Sondheim to correct his own heart condition. Rescuing Sondheim and defeating Cross, Lang was relieved when Sondheim was able to save the life of his daughter.
Lang intended to return the Ant-Man costume to its owner and turn himself in, but Henry Pym offered to let him keep them, provided Lang put them to lawful use. Since that time, Lang had donned the Ant-Man costume on various occasions to assist heroes such as Iron Man, the Avengers, or the law, although he considered himself a part-time crime fighter. During this time, Lang was instrumental in helping the Wasp defeat the Absorbing Man and Titania, two villains much stronger and more powerful than them. Often trying to keep his identity secret from his daughter, Cassie, Lang was eventually relieved to know that she discovered his secret on her own.
When Stark International was bought out by Stane International, Lang resigned out of respect to Tony Stark, the company's former head (and secretly Iron Man.) Lang opened his own electronics business. Later, he temporarily gave up the business to be hired by the Fantastic Four when their scientist and leader Reed Richards was missing for an extended period. Lang and Cassie lived at the Four's headquarters during this time.
During a time when many of Earth's heroes were believed dead after battling the psychic entity, Onslaught, Lang agreed to join the newly-reformed superteam Heroes for Hire. Lang returned to his electronics business when Heroes for Hire disbanded soon after Earth's heroes returned.
Lang continued to aid his superpowered colleagues as needed, such as joining other Avengers associates in infiltrating the base of the Avengers' enemies of the Triune Understanding, although this turned out to be a ruse by the Triunes to discredit the Avengers. He also helped the Avengers against the threat of the villain Kang the Conqueror, who nearly succeeded in conquering the world. After overthrowing Kang's would-be regime, the Avengers offered Lang a place on the roster, and he agreed.
Lang's place with the Avengers was not without its personal struggles. He began dating private investigator Jessica Jones, but the relationship ended when she revealed that she was pregnant with Luke Cage's baby. Lang was brought into court for a custody battle with his ex-wife over the safety of his daughter Cassie when she visited the Avengers Mansion headquarters of the team. Lang also struggled with his new teammate Jack of Hearts, as the two often clashed over trifling matters. However, when Lang's daughter was randomly kidnapped by a madman, Jack of Hearts rescued Cassie at the risk of his own powers consuming him, which ultimately killed him.
Lang felt guilty that he had only begun to reconcile with his new friend. Later, Jack of Hearts somehow returned to the Mansion with a message of warning for the team. When Lang reached out to his friend, Jack of Hearts detonated himself, taking half of the Avengers Mansion and seemingly Scott Lang with him. This attack was later discovered to be motivated by the Scarlet Witch, whose mind had become unhinged due to the magical powers she constantly felt forced to control. Whether this was truly the corpse of Jack of Hearts or just a magical construct of the Scarlet Witch remains unclear.
A few years later, the Young Avengers traveled back in time with an amnesiac Scarlet Witch to the moments before Avengers Mansion was destroyed and escaped along with Lang (leaving the Avengers of that time to believe he had perished). This celebration is short lived however, after Lang witnesses the death of his daughter Stature in battle with Dr. Doom. Lang has since sworn vengeance on Doom.
When the original Fantastic Four planned a trip through time and space, they decided to call four replacements just in case something would happen. Mr. Fantastic's replacement as the leader of the Fantastic Four and the Future Foundation was Scott, and the four original members indeed got lost in their adventure.
Pym Particles Ant-Man is able to reduce himself to the size of an ant, approximately one-half inch in height, by means of a rare group of sub-atomic particles, the source of which is as yet unknown, which he is able to contain in magnetic-field "canisters" (the shape of the magnetic lines of force were made visible by the confined particles, whose concentration caused them to behave like a gas).
These particles, whose wavelength is in the mental range when released, allow Ant-Man to will himself and other objects to reduce in size or enlarge. The mechanics involved having most of the mass of the nucleus of each affected atom convert into energetic particles which take up orbit around the nucleus - thus, although gravitons do not affect these particles, the mass of the original volume remains intact.
This means a punch delivered by the half-inch high Ant-Man would feel like the blow of a normal-sized man. Ant-Man's reducing particle has no time limit to its potency.
Lang has demonstrated the ability to change sizes without application of Pym particle gas.
Ant-Man's Helmet: Ant-Man also wore a cybernetic helmet designed by Henry Pym permitting him rudimentary communication and control of insects. He could broadcast to a range of about one mile (1.6 km), depending on the materials surrounding him. His helmet also contained sound amplification equipment which also shifted the frequency of his voice so that he could be heard by normal-sized humans despite his diminutive size, as well as providing its own oxygen supply.
When exposed to the "Horton Cells" while travelling Android Human Torch's body developed pyrokinetic powers. However the exposure was temporary. 
Ant-Man's Ants: Ant-Man customarily rode flying ants for transportation.
Avengers Quinjet: During his time with the Avengers, he also travelled by Quinjet.
Fantasti-Car: During his time with the Fantastic Four, he also travelled by Fantasti-Car.
Ant-Man's Wrist Gauntlets Ant-Man's wrist gauntlets enabled him to fire bio-electric blasts of adjustable force. His original helmet was able to fire 'disruptor' blasts of concussive force that Lang said were less powerful than the stings of The Wasp.