The counterpropagation of two laser beams through wave mixing in nonlinear optics may lead to spatiotemporal structures in the transverse beam profiles. We theoretically investigate the self-organization process of structures that arise in a photorefractive wave-mixing configuration with an external feedback mirror. The characteristic features mediated through the wave interaction in a bulk medium are discussed. Our group developed a beam propagation method that enabled us to perform numerical simulations beyond the first instability threshold. Primary and secondary spatiotemporal patterns, caused by the sluggish temporal response of the crystal in building reflection gratings, are observed. Analytically a GinzburgLandau equation for the order parameter and the corresponding longitudinal eigenfunctions of transverse modes, governing the propagation of the structures through the crystal, are derived and compared with our numerical results in one transverse dimension. First results of hexagonal patterns in two transverse dimensions are also presented.