[en] It is apparent that if higher energy accelerators are to be built for a reasonable cost, higher accelerating gradients will be required, and most efforts have been devoted to achieving such gradients. It must also be kept in mind, however, that higher gradients will require higher peak powers from whatever power source is used. The peak powers required are reduce if the wavelength of the accelerating radiation is reduced. If useful luminosity is to be achieved high average power is also needed. Average power is reduced if the beam emittances can be reduced and smaller interacting spots generated. The average power is also reduced if bunches containing larger numbers of particles are employed, but these numbers are limited by the beamstrahlung (synchrotron radiation) at the interaction point. Considerable progress has been made in understanding these questions, but much still remains to be studied. As an introduction to this subject, the limits on accelerating fields are examined as a function of wavelength and attempt to place the different schemes on such a plot. The constraints applied by beamstrahlung considerations and their influence on the choice of mechanisms are then examined