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4th Generation Light Sources will have brilliance performances which will exceed those of the 3rd Generation Light Sources by 10 orders of magnitude in peak value. 3rd Generation Light Sources were based on storage ring. Those sources had improved the quality of the X-Ray produced with respect to the 2nd generation of machines by reducing the emittance and by increasing the current of the stored beam. The great stability and small emittance were intrinsically obtained by the damping produced by the radiation emission itself. In 4th generation sources, the X-Ray source quality (brilliance, peak brilliance, coherence) will directly depend on the quality of the injected beam (emittance, peak current, average current). Also, its stability will primarily depend on that of their injector. 4th generation sources include both X-Ray FELs and ERL based sources. The technological challenges of injectors for X-Ray FELs include small emittances, high peak current, high stability and reliability. ERL based sources aim at the same type of performances, but in addition average current as high as those of the 3rd generation light sources is desired. The focus of this review will be on the technological challenges of X-Ray FELs sources but for solutions proposed by the ERL injector community which could benefit X-Ray FELs sources. Photoinjectors are the primary source choice for many of the X-Ray FELs under design and construction. Critical issues for this technology include optimum laser pulse shaping and high quality of emission from the photocathode. Beam performances obtained from photoInjectors up to date just fulfill the requirements of X-Ray FEL drivers, but adequate stability and reliability remain to be demonstrated. Alternate technologies to X-Ray FEL sources will also be briefly discussed.
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