  The major thrust of the Tully-Fisher (TF) surveys of distant galaxies is the
measurement of linewidths rather than mere redshifts or colors. Linewidths are
a measure of galaxy mass and should therefore be a more stable indicator of
size than galaxy brightness, which can be badly affected by luminosity
evolution. Masses may provide the best way to relate galaxies at different
epochs, but for such a program to work, we must control systematic effects that
could bias linewidth measurements at high redshift and skew comparisons with
local Tully-Fisher calibrations. Potential sources of confusion in TF studies
of galaxy structure and evolution include central or extended star bursts,
infalling gas, turbulence and outflows, dust extinction, calibration of
emission linewidths, and improper application of local TF calibrations to high
redshift galaxies.
