  I highlight recent progress in our understanding of the origin of disk galaxy
scaling laws in a hierarchically clustering universe. Numerical simulations of
galaxy formation in Cold Dark Matter (CDM) dominated universes indicate that
the slope and scatter of the I-band Tully-Fisher (TF) relation are well
reproduced in this model, although not, as proposed in recent work, because of
the cosmological equivalence between halo mass and circular velocity, but
rather as a result of the dynamical response of the halo to the assembly of the
luminous component of the galaxy. The zero-point of the TF relation is
determined mainly by the stellar mass-to-light ratio ($\Upsilon_I$) as well as
by the concentration ($c$) of the dark halo. For $c \sim 10$, as is typical of
halos formed in the ``concordance'' $\Lambda$CDM model, we find that this
requires $\Upsilon_I \sim 1.5$, in reasonable agreement with the mass-to-light
ratios expected of stellar populations with colors similar to those of TF
galaxies. This conclusion supersedes that of Navarro & Steinmetz (2000a,b), who
claimed the $\Lambda$CDM halos were too concentrated to be consistent with the
observed TF relation. The disagreement can be traced to inconsistencies in the
normalization of the power spectrum used in that work. Our new results show
that simulated disk galaxies in the $\Lambda$CDM scenario are not clearly
inconsistent with the observed I-band Tully-Fisher relation. On the other
hand,their angular momenta is much lower than observed. Accounting
simultaneously for the spin, size and luminosity of disk galaxies remains a
challenge for hierarchical models of galaxy formation.
