  We investigate the nature of Ultra Faint dwarf spheroidal galaxies (UF dSphs)
in a general cosmological context, simultaneously accounting for various
"classical" dSphs and Milky Way (MW) properties, including their Metallicity
Distribution Function (MDF). The model successfully reproduces both the
observed [Fe/H]-Luminosity relation and the mean MDF of UFs. According to our
results UFs are the living fossils of H2-cooling minihaloes formed at z>8.5,
i.e. before the end of reionization. They are the oldest and the most dark
matter-dominated (M/L > 100) dSphs in the MW system, with a total mass of M =
10^(7-8) Msun. The model allows to interpret the different shape of UFs and
classical dSphs MDF, along with the frequency of extremely metal-poor stars in
these objects. We discuss the "missing satellites problem" by comparing the UF
star formation efficiencies with those derived for minihaloes in the Via Lactea
simulation.
