Astrophysical masers are natural stimulated emission sources emitted from
various astrophysical environments. I briefly discuss the environments and
the molecular carriers of maser spectral lines. I consider the use of masers
as tools in astronomy for the measurement of magnetic fields, Galactic and
cosmological distances and mass estmation. I then introduce my own research,
including 3D modelling of maser flares, arising from cloud rotation, variable
pump and background rotation, shock impact and line-of-sight overlap.