The atmospheres of upper mainsequence chemically peculiar stars (ApBp stars) are known to be strongly affected by atomic diffusion. In recent years, both observations and models have shown that, as expected, the classical picture of abundance stratification is far too simple. A new generation of observational and modelling tools is appearing to take into account the complexity of the situation. In this talk, I will give an overview on recent results concerning the models for ApBp stars atmospheres. I will evaluate the two main approaches to estimate quantitatively the abundance stratifications: those using the so-called equilibrium hypothesis vs. those calculating time-dependent diffusion.