  Fast Radio Bursts (FRBs), with a typical duration of 1 ms and 1 Jy flux
density at GHz frequencies, have brightness temperatures exceeding 1e33 K,
requiring a coherent emission process. This can be achieved by bunching
particles in volumes smaller than the typical wavelength, but this may be
challenging. Alternatively, we can have maser emission. Under certain
conditions, the synchrotron stimulated emission process can be more important
than true absorption, and a synchrotron maser can be created. This occurs when
the emitting electrons have a very narrow distribution of pitch angles and
energies. This process overcomes the difficulties of having extremely dense
bunches of particles and relaxes the light crossing time limits, since there is
no simple relation between the actual size of the source and the observed
variability timescale.
