  The physical background of scaling laws of disk galaxies is reviewed. The
match between analytically derived and observed scaling laws is briefly
discussed. Accurate modeling of the fraction of baryons that end populating a
disk, and the conversion efficiency of those into stars, remains a challenging
task for numerical simulations. The measurement of rotational velocity tends to
be made with criteria of convenience rather than through rigorous definition.
And yet, the Tully-Fisher and the disk size vs. rotational velocity relations
exhibit surprisingly low scatter. Practical recipes (and costs) to optimize the
quality of template relations are considered.
