Leonid Keldysh (7 April 1931 – 11 November 2016) was a Russian physicist. Keldysh was a professor in the I.E. Tamm Theory division of the Lebedev Physical Institute of the Russian Academy of Sciences in Moscow and a faculty member at Texas A&M University. He was known for developing the Keldysh formalism, a powerful quantum field theory framework designed to describe a system in a non-equilibrium state, as well as for the theory of (Keldysh-Kopaev model, with ). Keldysh's awards includethe 2009 Rusnanoprize, an international nanotechnology award, for his work related to molecular beam epitaxy,the 2011 Eugene Feenberg Memorial Medal, and the 2015 Lomonosov Grand Gold Medal of the Russian Academy of Sciences.