Pattern formation and competition occur in a nonlinear extended medium if d
issipation allows for attracting sets, independently of initial and boundar
y conditions. This intrinsic patterning emerges from a reaction diffusion d
ynamics (Turing chemical patterns). In optics, the coupling of an electroma
gnetic field to a polarizable medium and the presence of losses induce a mo
re general (diffraction-diffusion) mechanism of pattern formation. The pres
ence of a coherent phase propagation may lead to a large set of unstable ba
nds and hence to a richer variety with respect to the chemical case. A revi
ew of different experimental situations is presented, including a discussio
n on suitable indicators which characterize the different regimes. Vistas o
n perspective new phenomena and applications include an extension to atom o
ptics. (C) 1999 Elsevier Science B.V. All rights reserved.