Thomas Callister Hales (born June 4, 1958) is an American mathematician working on the Langlands program. He is known in the area for having worked on the fundamental lemma, and proving a special case of it over the group Sp(4). Many of his ideas were incorporated into the final proof, due to Ngô Bảo Châu. He is also known for his 1998 computer-aided proof of the Kepler conjecture, a centuries-old problem in discrete geometry which states that the most space-efficient way to pack spheres is in a pyramid shape. Hales also proved the honeycomb conjecture.