The manipulation of beam phase space has been proposed as a method to
overcome the degradation of FEL performance from beam emittance
(transverse beam temperature). Consequences of beam conditioning, could
it realized, include a significant reduction (factor of ~2) in beam
energy needed for FELs to reach hard x-ray wavelengths. The theory of
beam conditioning will be presented along with a discussion of
difficulties and possible methods to realize it.