John Werner Cahn is an American scientist and recipient of the 1998 National Medal of Science. He was a professor in the department of Materials Science at MIT from 1964 to 1978. He was born on born January 9, 1928 in Cologne, Germany. Cahn received a bachelor's degree in Chemistry in 1949 from the University of Michigan. He later earned a Ph.D in Physical Chemistry in 1953 from the University of California at Berkeley. In 1964, Cahn became a professor in the Department of Metallurgy at the Massachusetts Institute of Technology. He left MIT in 1978. In 1969, Cahn began a long professional relationship with his graduate student Francis Larche, whose work focussed on the effect of mechanical stress on the thermodynamics of solids. The Larche Cahn approach is the cornerstone of the treatment of the thermodynamics of stressed materials. Good examples of this phenomenon are the regions near a coherent precipitate or the stress field around a dislocation.