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Photorefractive materials have been assuming ever increasing prominence in the non-linear optics field due to their unique capability for displaying strong non-linear effects at milliwatt power levels. In this review we describe in some detail the two currently accepted models of the photorefractive process. The “band transport” model assumes that photo excited electrons (or holes) are ejected from filled donor (or acceptor) sites to the conduction (or valence) band where they migrate to dark regions in the crystal before recombining into empty traps. The charge separation results in a space charge field which modulates the refractive index via the linear electro-optic effect. The “hopping” model assumes that carrier transport occurs via light induced hopping from a filled donor site to a neighbouring empty trap. In particular, we examine the timeand spatial frequency dependence of grating formation and discuss the …