Many out-of-equilibrium systems undergo an instability when an order
parameter is suitably tuned, giving rise to well known and widespread 
processes: growth, convection, phase separation, and so on. A common 
feature is the emergence of a nontrivial spatial structure, whose
typical scale L may display different dynamics. We are specially
interested in understanding when L increases in time (coarsening process) and how. 
Using a continuum modelling and a multiscale approach, for several
equations we are able to obtain a phase diffusion equation describing 
the dynamics of the emerging pattern and to predict when and how coarsening proceeds.