 The correspondence principle was first introduced in the context of the old quantum theory, which was developed in the period between 1900 and 1925. The old quantum theory was an “interim” theory developed in response to the recognition that Newtonian mechanics and classical electrodynamics are inadequate for the description of atomic systems. Although the old quantum theory had remarkable empirical successes, it was criticized for having what many thought were inconsistent foundations. The old quantum theory was ultimately replaced by modern quantum mechanics with Werner Heisenberg’s development of matrix mechanics in 1925 and Erwin Schrödinger’s development of wave mechanics in 1926.