  Physics of counterpropagating optical beams and spatial optical solitons is
reviewed, including the formation of stationary states and spatiotemporal
instabilities. First, several models describing the evolution and interactions
between optical beams and spatial solitons are discussed, that propagate in
opposite directions in nonlinear media. It is shown that coherent collisions
between counterpropagating beams give rise to an interesting focusing mechanism
resulting from the interference between the beams, and that interactions
between such beams are insensitive to the relative phase between them. Second,
recent experimental observations of the counterpropagation effects and
instabilities in waveguides and bulk geometries, as well as in one- and
two-dimensional photonic lattices, are discussed. A variety of different
generalizations of this concept are summarized, including the
counterpropagating beams of complex structures, such as multipole beams and
optical vortices, as well as the beams in different media, such as
photorefractive materials and liquid crystals.
