  Spatial pattern formation in excitable fluctuating media was researched
analytically from the point of view of the order parameters concept. The
reaction-diffusion system in external noise is considered as a model of such
medium. Stochastic equations for the unstable mode amplitudes (order
parameters), dispersion equations for the unstable mode averaged amplitudes,
and the Fokker-Planck equation for the order parameters have been obtained. The
developed theory makes it possible to analyze different noise-induced effects,
including the variation of boundaries of ordering and disordering phase
transitions depending on the parameters of external noise
