  We discuss possible origins of scaling laws relating structural properties of
disk galaxies within the context of hierarchically clustering theories of
galaxy formation. Using gasdynamical simulations that incorporate the effects
of star formation we illustrate these global trends and highlight the
difficulties faced by models that envision disk galaxies as the final outcome
of a hierarchical sequence of merger events. In particular, we focus on the
cosmological origin of the Tully-Fisher relation, and argue that this
correlation between the total luminosity and rotation speed of disk galaxies is
a natural result of the approximately scale free formation process of the
massive halos they inhabit. Although the slope and scatter of the Tully-Fisher
relation can be readily reproduced in hierarchical formation scenarios, the
observed zero-point of the relation is inconsistent with simulations of galaxy
formation in Cold Dark Matter universes, a difficulty that can be traced to the
high central mass concentration of dark halos formed in this cosmogony. This
result indicates that substantial revision to the new ``standard'' model of
structure formation ($\Lambda$CDM) may be needed in order to accommodate
observations on the scale of individual disk galaxies.
