Gamma Radiation

Gamma Radiation was being tested by Bruce Banner when he was exposed turning him into The Hulk.

The Incredible Hulk
Bruce Banner experiments with gamma radiation and then turns into the Hulk.

Thor
Erik Selvig mentions a scientist who experimented with gamma radiation.

The Avengers
The Tesseract is mentioned to emit traces of gamma radiation, leading to the recruitment of Bruce Banner to assist in finding it.