From Mother Jones:
by Mark Murrmann
In the early 1900s, Lewis Hine left his job as a schoolteacher to work as a photographer for the National Child Labor Committee, investigating and documenting child labor in the United States. As a sociologist, Hine was an early believer in the power of photography to document work conditions and help bring about change. He traveled the country, going to fields, factories, and mines—sometimes working undercover—to take pictures of kids as young as four years old being put to work.
See the full photo-set from Mother Jones.