  We present here the final results of the Hubble Space Telescope Key Project
to measure the Hubble constant. We summarize our method, the results and the
uncertainties, tabulate our revised distances, and give the implications of
these results for cosmology. The analysis presented here benefits from a number
of recent improvements and refinements, including (1) a larger LMC Cepheid
sample to define the fiducial period-luminosity (PL) relations, (2) a more
recent HST Wide Field and Planetary Camera 2 (WFPC2) photometric calibration,
(3) a correction for Cepheid metallicity, and (4) a correction for
incompleteness bias in the observed Cepheid PL samples. New, revised distances
are given for the 18 spiral galaxies for which Cepheids have been discovered as
part of the Key Project, as well as for 13 additional galaxies with published
Cepheid data. The new calibration results in a Cepheid distance to NGC 4258 in
better agreement with the maser distance to this galaxy. Based on these revised
Cepheid distances, we find values (in km/sec/Mpc) of H0 = 71 +/- 2 (random) +/-
6 (systematic) (type Ia supernovae), 71 +/- 2 +/- 7 (Tully-Fisher relation), 70
+/- 5 +/- 6 (surface brightness fluctuations), 72 +/- 9 +/- 7 (type II
supernovae), and 82 +/- 6 +/- 9 (fundamental plane). We combine these results
for the different methods with 3 different weighting schemes, and find good
agreement and consistency with H0 = 72 +/- 8. Finally, we compare these results
with other, global methods for measuring the Hubble constant.
