Operation of cyclotron resonance masers (CRMs) at slow electromagnetic (EM) waves has a number of potential benefits. Among them is, first, the possibility of highly efficient operation due to extraction of energy from both orbital and axial motion of electrons. Since in the case of slow EM waves the axial bunching of electron dominates over the orbital one  it is possible to achieve a high efficiency even with a relatively small orbital to axial velocity ratio, a (note that such low a beams are more stable with respect to various space charge instabilities in the region between the electron gun and the interaction space). Second, it follows from the cyclotron resonance condition, w nil/(1 - 1//3,) (here w and h = kzelw are, respectively, the frequency and normalized axial wavenumber of the EM wave, nil is the resonant harmonic of the cyclotron frequency, and 13, is the axial electron velocity normalized to the speed of light), that when h > 1 (slow EM wave) it is possible to realize a high Doppler frequency up-conversion even operating with moderately relativistic electron beams. In addition, at a given operating frequency one needs lower magnetic fields than in the case of fast waves. In some cases, this would eliminate the need for superconducting magnets. The third benefit is the absence of an intersection between the beam line and the operating mode at lower frequency, either near cut-off or as an opposite wave, since in slow-wave CRMs the beam line w = nil + Iczcz may intersect the dispersion curve w(lcz) only at one point. This benefit, which makes operation at the desired mode more stable, as well as the opportunity to operate in a wide frequency band, is especially important for CRM-amplifiers 121.