  We present new metallicity measurements for 298 individual red giant branch
stars in eight of the least luminous dwarf spheroidal galaxies (dSphs) in the
Milky Way (MW) system. Our technique is based on medium resolution Keck/DEIMOS
spectroscopy coupled with spectral synthesis. We present the first
spectroscopic metallicities at [Fe/H] < -3.0 of stars in a dwarf galaxy, with
individual stellar metallicities as low as [Fe/H] = -3.3. Because our [Fe/H]
measurements are not tied to empirical metallicity calibrators and are
sensitive to arbitrarily low metallicities, we are able to probe this extremely
metal-poor regime accurately. The metallicity distribution of stars in these
dSphs is similar to the MW halo at the metal-poor end. We also demonstrate that
the luminosity-metallicity relation previously seen in more luminous dSph
galaxies (M_V = -13.4 to -8.8) extends smoothly down to an absolute magnitude
of M_V = -3.7. The discovery of extremely metal-poor stars in dSphs lends
support to the LCDM galaxy assembly paradigm wherein dwarf galaxies dissolve to
form the stellar halo of the MW.
