Is an x-ray maser possible? I was wondering if lasers evolved from masers. They applied the same principles to a different wavelength.
Would it be possible to create a maser at a higher wavelength, using x-rays, gamma rays, etc.?
 A: As Martin Beckett pointed out, x-ray lasers do exist. One important limitation to make them more similar to the common lasers at longer wavelength is the absence of mirrors which can reflect back x-rays (only grazing incidence is possible with short wavelengths). So a standard optical cavity cannot be made, and this prevents the feedback and amplification by many passages in the cavity as in common lasers. 
As far as I know, all the X-ray lasers are single pass, i.e. a X-ray pulse (~1ps) travels and gets amplified by stimulated emission in a transiently-inverted medium, a plasma, only once and then leaves. One optical technique to obtain the active plasma is Chirp Pulse Amplification.
The x-ray pulse starts initially from spontaneous emission noise, so combining this with the single pass amplification makes the output poorly coherent.
(At least) one trick has been demonstrated to have saturated gain and full coherence in the output: injecting in the plasma amplifier a small seed of coherent photons with the correct wavelength (obtained by High Harmonic Generation), the characteristics of the output light are comparable with those of common lasers. (This is also mentioned at the end of the article linked by Martin Beckett)
A: There are a few different ways of operating x-ray lasers, and quite a few in operation.
http://www.osa-opn.org/home/articles/volume_19/issue_5/features/the_history_of_the_x-ray_laser/
If these are really lasers, with a population inversion and stimulated emission - rather than just mechanisms to create very brief pulses of coherent radiation is probably arguable.
