  The focus of this review is the work that has been done during the 1990s on
using Type Ia supernovae (SNe Ia) to measure the Hubble constant ($H_0$). SNe
Ia are well suited for measuring $H_0$. A straightforward maximum-light color
criterion can weed out the minority of observed events that are either
intrinsically subluminous or substantially extinguished by dust, leaving a
majority subsample that has observational absolute-magnitude dispersions of
less than $\sigma_{obs}(M_B) \simeq \sigma_{obs}(M_V) \simeq 0.3$ mag.
Correlations between absolute magnitude and one or more distance-independent SN
Ia or parent-galaxy observables can be used to further standardize the absolute
magnitudes to better than 0.2 mag. The absolute magnitudes can be calibrated in
two independent ways --- empirically, using Cepheid-based distances to parent
galaxies of SNe Ia, and physically, by light curve and spectrum fitting. At
present the empirical and physical calibrations are in agreement at $M_B \simeq
M_V \simeq -19.4$ or -19.5. Various ways that have been used to match
Cepheid-calibrated SNe Ia or physical models to SNe Ia that have been observed
out in the Hubble flow have given values of $H_0$ distributed throughout the
range 54 to 67 km/s Mpc$^{-1}$. Astronomers who want a consensus value of $H_0$
from SNe Ia with conservative errors could, for now, use $60 \pm 10$ km/s
Mpc^{-1}$.
