  We discuss several qualitative properties of the solutions of
reaction-diffusion systems and equations of the form $u_t = \epsilon^2 D \Delta
u + f(u,x,\epsilon t)$, that are used in modeling pattern formation. We analyze
the diffusion neutral and the diffusion dependent situations that, in the time
autonomous case, are distinguished by considering the attractors of the shorted
equation $u_t = f(u,x)$. We discuss the consequences of being in one or in the
other of the two situations and present examples from developmental biology and
from fluid mechanics.
