The Hulk is the form taken by Bruce Banner whenever he is stressed, enraged or seriously injured. This is due to exposure to gamma radiation during an experiment attempting to find a way of making humans immune to gamma radiation.

Dr. Bruce Banner is a genius in nuclear physics, possessing a mind so brilliant that it cannot be measured on any known intelligence test. When Banner is the Hulk, Banner's consciousness is buried within the Hulk's, and can influence the Hulk's behavior only to a very limited extent.

The Hulk possesses an incredible level of superhuman physical ability. His capacity for physical strength is potentially limitless due to the fact that the Hulk's strength increases proportionally with his level of great emotional stress, anger in particular. The Hulk uses his superhumanly strong leg muscles to leap great distances. The Hulk has been known to cover hundreds of miles in a single bound and once leaped almost into orbit around the Earth.

Robert Bruce Banner was the son of an alcoholic who deeply hated him. Banner's mother showed much affection for her child, who returned her love, but this only served to fuel his father's rage. Dr. Brian Banner was an atomic physicist who worked on producing clean nuclear power as an energy source, but he was afraid his exposure to it mutated his son's genes. Bruce showed signs of high intelligence at an early age which strengthened his father's belief. Brian became abusive to Bruce and when his mother intervened, Brian murdered her.

Supervising the trial of an experimental gamma bomb that he designed for the U.S. Defense Department at a nuclear research facility in New Mexico, Bruce selflessly rushed to the rescue of an ignorant teenager who had wandered onto the testing field as the countdown ticked inexorably toward zero.

At first, Banner would transform into the brutish gray Hulk only at sunset, and revert to human form at dawn. Eventually, his changes into the childlike green Hulk came to be triggered by the release of adrenaline when he was intensely excited, no matter what time of day.