The number of small-size highly stable sources of electromagnetic radiation has been developed for the millimeter wave range. For input power of 500 W and magnet clearance of 32 mm, the output power is of 0.5...50 W, the noise level is 20 dB lower than with carcinotron or klystron. Over the past three decades, systematic investigations on development of vacuum-electronic devices of a new class, namely, diffraction radiation generators (DRG) have been in progress in the Institute of Radiophysics and Electronics of the Academy of Sciences of Ukraine. Efficient excitation of millimeter-wave electromagnetic oscillations in DRG owes to the Smith-Purcell radiation , or diffraction radiation occurring when electron beam skims diffraction grating placed on the reflection surface of one of the mirrors of open resonator (OR) . The key feature of DRG from classical vacuum-electronic devices, apart from its comparatively high power, is the better short-term stability of frequency. Frequency instability of DRG is ',:,d10-10 per tens of microseconds, noise level is 20 dB lower than with carcinotron or klystron with the same supply . Extensive experiments and approximate computational methods were the basis for the design of DRG operating over 30...375 GHz. For the whole millimeter-wave range to be spanned, the set of small-size highly stable DRG packaged into the optimal magnet systems with air clearance of about 32 mm was composed . A tuning range of each device of the set is about of 20%, attainable power and efficiency vary as the square of each device minimum wavelength. With input power less than 500 W 2.4...4.0 kV, I< 0.13 A) the output power is up to 50 W at a frequency of 30 GHz and 0.5 W at 300 GHz . With more powerful electronic sources the output power may be essentially increased. The research into continuous-wave, reflection, pulse, autodyne and other special modes of DRO was taken up to create the devices which, found their use in DRG-spectroscopy, DRG-location, DRG-autodyne studies of plasma and bioobjects as well as in DRG- pumping of dynamic polarized nuclear targets . For the further development of vacuum-electronic devices we focus on maximization of efficiency and output power of DRG, provision of superhighly stable oscillations in DRG, design of wide-band DRG with effective control for power and frequency, development of physical principles and techniques for creation of new diffraction-electronic devices with extended functionalities. Improvement of long-term stability of DRG-frequency may extend essentially its application field. Of practical interest is investigation of DRG mode locking achieved with a lower-frequency driving oscillator of low power and high stability. With advance to the short-wavelength region all the problems become more complicated. But DRG provides the greater scope for their solution than the classical electronic devices. DRG offers essentially increased interaction space and diverse possibilities for forming optimal highly stable fields by varying the type and dimensions of OR mirrors. Spatially developed diffraction gratings and "thick" electron beams with longitudinally-transverse interaction are promising.