The problem of detecting feeble radio signals of short wavelengths, always important in obvious defense applications, has become crucial to the study of certain astrophysical questions by means of radio astronomy. Hitherto, the lower limit on the strength of a usefully detectable signal has been the background noise generated by the more or less “hot” electrons in the vacuum tube amplifiers or crystal detectors at the head of the receiver. This difficulty has led to suggestions for replacing these more conventional input stages by atomic resonant systems, which should be much less noisy. The most famous of these are the so‐called “masers” (Microwave Amplification by Stimulated Emission of Radiation): the atomic beam maser of Townes, Gordon, and Zeiger, and the “Solid‐State Maser” of N. Bloembergen which uses a paramagnetic impurity concentration in a solid as resonant system. For purposes of comparison as well as by way of introduction, we shall begin with a simple statement of their common operating principle.