  Wavy film flow of incompressible Newtonian fluid down an inclined plane is
considered. The question is posed as to the parametric conditions under which
the description of evolution can be approximately reduced for all time to a
single evolution equation for the film thickness. An unconventional
perturbation approach yields the most general evolution equation and least
restrictive conditions on its validity. The advantages of this equation for
analytical and numerical studies of 3D waves in inclined films are pointed out.
