The Four Color Theorem and its aftermath

The Four Color Theorem is known to be one of the most famous results in graph theory. This theorem was proved by Appel and Haken in 1977, but the proof was computer-assisted to deal with about 2000 distinct cases. The statement of the Four Color Theorem is so simple, but its proof required the complicated arguments with computer. We consider whether we can persuade this situation, and give an explanation of the Four Color Theorem, introducing several results and notions derived from it, and moreover, recent developments related to it.