  We present a phenomenological approach to the study of disk galaxy evolution,
based on i) a detailed modelling of the Milky Way (used as a prototype disk
galaxy) and ii) an extension of the model to other disks through some simple
scaling relations, obtained in the framework of Cold Dark matter models. The
main conclusion is that, on average, massive disks have formed the bulk of
their stars earlier than their lower mass counterparts. It is not yet clear why
the ``star formation hierarchy'' has been apparently opposite to the ``dark
matter assembly'' hierarchy.
