Leonard Susskind is an American theoretical physicist, professor of theoretical physics at Stanford University, and founding director of the Stanford Institute for Theoretical Physics. His research interests include string theory, quantum field theory, quantum statistical mechanics, and quantum cosmology. He is a member of the US National Academy of Sciences and the American Academy of Arts and Sciences. Susskind is also an associate member of the faculty of Canada's Perimeter Institute for Theoretical Physics and a distinguished professor of the Korea Institute for Advanced Study.

He is widely regarded as one of the fathers of string theory. Susskind was the first to give a precise string-theoretic interpretation of the holographic principle in 1995 and introduced the idea of the string theory landscape in 2003.

In recognition of his contributions, he was awarded the 1998 J. J. Sakurai Prize, the 2018 Oskar Klein Medal, and the Dirac Medal of the International Centre for Theoretical Physics in 2023.