Principles of design and construction are described for modern quantum amplifiers (masers), which are the most sensitive input devices of ultrahigh-frequency radio receiving systems. Data are given on the characteristics of paramagnetic crystals for quantum amplifiers (QA) and on the attained QA parameters. Problems are discussed of application of QA in radio astronomy (cosmic radiospectroscopy, radar observation of planets, very-long-baseline radiointerferometry, etc.) and in long-range space radio communication systems.