The theory of optical dispersive shocks generated in propagation of light beams through photore-
fractive media is developed. Full one-dimensional analytical theory based on the Whitham modu-
lation approach is given for the simplest case of sharp step-like initial discontinuity in a beam with
one-dimensional strip-like geometry. This approach is con¯rmed by numerical simulations which are
extended also to beams with cylindrical symmetry. The theory explains recent experiments where
such dispersive shock waves have been observed.