Stern has developed the method of molecular rays into a powerful tool for the investigation of the properties of ultimate particles. His first application was the experimental verification of Maxwell's law of velocity distribution in gases. Then followed his famous work, in collaboration with Gerlach, on the deflexion of atoms by the action of an inhomogeneous magnetic field on the atom's magnetic moment; this provided direct evidence for one of the strangest statements of quantum mechanics, the so-called quantization of direction. By an almost incredible refinement of this method, Stern succeeded in detecting and measuring the (about 2,000 times smaller) magnetic moments of some nuclei, the proton and the deuteron. It is this work for which he has now been awarded a Nobel Prize. But he used his method also for other purposes. He gave the most striking proof for the wave nature of ordinary matter, as formulated by de Brogue, in producing interferences by rays of ordinary matter, hydrogen and helium, reflected by crystal surfaces. Stern has also published important theoretical papers on thermodynamics and quantum theory. He was professor of physical chemistry at Hamburg, and when he was compelled to leave Germany in 1933 he became a member of the staff of the Carnegie Institute of Technology, Pittsburgh, Pennsylvania.