Albert Einstein publishes in 1917 "Zur Quantentheorie der Strahlung" (On the Quantum Theory of Radiation), the formal journal reissue of a work presented the previous year before the Zurich Physical Society. In it he combines Bohr's atomic model (1913) with Planck's quantum hypothesis (1900) to derive, in an alternative and much simpler way, Planck's black-body radiation law. To achieve this, Einstein postulates that an atom in an excited energy level can emit a photon in two ways: spontaneously, or stimulated by the presence of another photon of the same frequency, in which case the emitted photon comes out in phase and in the same direction as the one that stimulated it. He introduces the A coefficient (spontaneous emission) and B coefficients (absorption and stimulated emission), thereby laying the complete theoretical foundation of stimulated emission of radiation. Einstein builds no device and predicts no concrete technological application; the paper is strictly theoretical. Almost forty years later, this stimulated-emission mechanism is the direct physical basis on which Charles Townes builds the maser (1953) and Theodore Maiman the laser (1960) — whose very name, "light amplification by stimulated emission of radiation", explicitly refers to the concept Einstein formalizes in this paper.