George Green was a baker and grain miller who became an important mathematician largely through self-study. He was born in England in 1793 and spent most of his life working in his family business while studying mathematics in his spare time. He had very little formal education, so it is not fully known how he learned advanced mathematics. Despite this, he developed ideas that later became important in mathematical physics. In 1828, he published an essay that introduced what we now know as Green’s theorem. In simple terms, Green’s theorem connects what happens along the boundary of a closed region with what happens throughout the area inside it. Instead of calculating certain quantities only along the boundary, we can calculate them by considering the corresponding quantities over the entire region. Green’s theorem is closely related to the divergence theorem and Stokes’ theorem. Green’s life also shows that important mathematical work can come from people working outside traditional academic settings.