 Callan is best known for his work on broken scale invariance (Callan–Symanzik equation) and has also made leading contributions to quantum field theory and string theory in the areas of dyon-fermion dynamics, string solitons and black holes.Callan has been a member of the JASON defense advisory group since 1968, and was chair of the group from 1990 to 1995.He served as president of the American Physical Society (APS) in 2010.