Time-dependent diffusion in A-stars
  Each time diffusion of elements is invoked in explaining abundance anomalies in a star, this supposes implicitly that a stratification process is in progress somewhere in that star. This means also, that the element abundances can still be evolving according to the star's age and fundamental parameters. Moreover, it has been shown that the superficial abundances may have complex temporal behavior. This should be detectable through new observations. In some cases, it may be already apparent in available data.   The building up of the elements' stratification is a very difficult process to study. This is due to the existence of strong non-linearities in the time-dependent equations, which must be solved numerically. We will discuss some works that tackle this problem for Ap and Am stars, and we will present some results concerning mostly Am stars. Future desirable improvements in these studies will be considered. 
Georges Alecian