**1.1. Peculiar Velocities versus H_{0}**

What it means to ``measure a galaxy's distance'' depends
on whether one is interested in studying peculiar
velocities or determining the value of the Hubble constant.
A galaxy's peculiar velocity may be estimated given its ``distance''
in km s^{-1} - the part of its radial velocity due solely
to the Hubble expansion. The same object provides an
estimate of *H*_{0} only if one can measure its distance
in metric units such as megaparsecs. What this means in practice is that
accurate peculiar velocity studies may be carried
out *today*, despite the fact that *H*_{0} remains
undetermined at the ~ 20% level.

Another basic distinction between velocity analysis and
the search for *H*_{0} concerns the distance regimes in which they
are optimally conducted. Peculiar velocity surveys are best
carried out in the ``nearby'' universe, where peculiar velocity errors
are comparable to or less than the peculiar velocities themselves.
The characteristic amplitude of the
radial peculiar velocity, *v _{p}*, is a few hundred km
s

On the other hand, to obtain the absolute distances
needed to measure *H*_{0}, we must first calibrate our distance
indicators *locally* ( 2000 km s^{-1}). This is because
the distance indicators capable of reaching the ``far field'' (
7000 km s^{-1}) of the Hubble flow generally have no *a priori*
absolute calibration (cf. Section 1.2).
The only reliable distance indicator that
can bridge the gap between the Milky Way and the handful of
Local Group galaxies whose absolute distances are well-known,
and galaxies beyond a few Mpc,
is the Cepheid variable method (Section 2), which
is limited to distances
2000 km s^{-1}. As a result, *Hubble
constant measurement is inherently a two-step process*: local
calibration in galaxies with Cepheid distances,
followed by distance measurements in the far field where
the effect of peculiar velocities is small.
The local calibration
step is unnecessary in peculiar velocity studies.

Although peculiar velocity surveys
and *H*_{0} measurement differ in the ways just
discussed, the two problems are, ultimately, closely related.
Many distance indicator methods have been
and are being used for both purposes. Indeed, a distance indicator calibrated
in km s^{-1} may be
turned into a tool for measuring *H*_{0}
simply by knowing the distances in Mpc to a few well-studied objects
to which it has been applied.
This Chapter will thus be organized not around the peculiar
velocity-*H*_{0} distinction, but rather around methods
of distance estimation.