Abstract: Employing the inner-shell transition mechanism is a potential way to obtain ultra-short wavelength lasers. It is not a hopeless dream to produce an ultra-short wavelength hard X-ray laser using this mechanism, in light of the recent great developments in ultra-fast and ultra-intense laser technology as well as experimental X-ray laser research. In this paper the principle of inner-shell X-ray lasers is introduced and some possible approaches to the realization of such lasers are discussed in detail.