Fat Man and Little Boy (aka Shadow Makers in the UK) is a 1989 film that reenacts the Manhattan Project, the secret Allied endeavor to develop the first nuclear weapons during World War II. It is named after the nuclear weapons known as "Fat Man" and "Little Boy", and also potentially as a reference to the portly Gen. Leslie R. Groves and the lithe Robert Oppenheimer, respectively the military and scientific heads of the project.

It stars Paul Newman as Gen. Leslie R. Groves, Dwight Schultz as Robert Oppenheimer, Laura Dern as nurse Kathleen Robinson, and John Cusack as young physicist Michael Merriman. The character of Merriman is a fictional composite of Harry K. Daghlian and Louis Slotin, two Los Alamos scientists who died in criticality accidents. (Contrary to Merriman's death in the movie, these accidents and deaths occurred after the dropping of the two bombs on Japan; see their respective articles.)

The film was directed by Roland Joffe and written by Joffe and Bruce Robinson. The film has been criticised for distortion for dramatic effect, and is also cited as an example of miscasting in its choices of Paul Newman, definitely not a portly actor, for the role of General Groves, and Dwight Schultz for the role of Oppenheimer. The film made under $4 million on its original release.