  The luminosity of the Tip of the Red Giant Branch (TRGB) provides an
excellent measure of galaxy distances and is easily determined in the resolved
images of nearby galaxies observed with Hubble Space Telescope. There is now a
large amount of archival data relevant to the TRGB methodology and which offers
comparisons with other distance estimators. Zero-point issues related to the
TRGB distance scale are reviewed in this paper. Consideration is given to the
metallicity dependence of the TRGB, the transformations between HST flight
systems and Johnson-Cousins photometry, the absolute magnitude scale based on
Horizontal Branch measurements, and the effects of reddening. The zero-point of
the TRGB is established with a statistical accuracy of 1%, modulo the
uncertainty in the magnitude of the Horizontal Branch, with a typical rms
uncertainty of 3% in individual galaxy distances at high Galactic latitude. The
zero-point is consistent with the Cepheids period-luminosity relation scale but
invites reconsideration of the claimed metallicity dependence with that method.
The maser distance to NGC 4258 is consistent with TRGB but presently has lower
accuracy.
