Lee Sedol was a professional player from South Korea who achieved the 9-dan rank in Go, an ancient board game that requires players to use strategic thinking and to surround more territory than their opponent. With 18 international championships to his name, Sedol, now retired, is recognized as one of the greatest players in the game.

In 2016, he participated in the Google DeepMind Challenge Match against AlphaGo, an artificial intelligence program developed by Google DeepMind. In the game, Lee's unexpected move at white 78 created a white wedge between the black stones in the center. The apparently embarrassed AlphaGo responded poorly on move 79, suddenly turning the game in Lee's favor. AlphaGo then declared its surrender by displaying a "resign" message on the computer screen. Ultimately, however, the AI emerged victorious in the series.