Underlying Quantum Mechanics is the theory that Planck posited and then verified that states: energy is not a smooth flowing continuum, but rather, it is manifested by the emission from radiating bodies of discrete particles called quanta.

Planck based this quantum theory on a discovery he made in 1900. He constructed a formula that related the energy of radiation to its frequency. This formula which defines the variable h is now known as Planck's radiation formula. The whole "quantum" concept required Planck to reconsider some basic principles, such as the Second Law of Thermodynamics (the entropy law). Planck's initial theory and the work carried on by other physicists, such as Niels Bohr and Paul Dirac, basically redefined the nature of physics.

Planck was 42 in 1900 when his original groundbreaking work was completed. Eighteen years later, in 1918, in recognition of his revolutionary quantum theory, Planck was awarded the Nobel Prize in physics. The latter part of his life was spent more in administrative roles (he was president of the Kaiser Wilhelm Society, later the Max Planck Society, from 1930-1937), and in confronting tragedy and disaster. In 1909 his wife, Marie Merck died. In 1916 his oldest son, Karl, was killed in action during World War One. In 1917, a daughter, Margarete died during childbirth. In 1919, his other daughter, Emma, died during childbirth. In 1944, Planck's house in Berlin was destroyed by enemy bombs. In 1945 his younger son, Erwin, having been implicated in the Hitler assassination attempt on July 20, 1944 died at the hands of the Gestapo. Two years later, having been relocated to Gottingen with his second wife by the American forces, Planck died at the age of 89. A colleague remarked that death came "as a redemption."