  We use a self-consistent model of galaxy formation and the evolution of the
intergalactic medium to study the effects of the reionization of the universe
at high redshift on the properties of satellite galaxies like those seen around
the Milky Way. Photoionization suppresses the formation of small galaxies, so
that surviving satellites are preferentially those that formed before the
universe reionized. As a result, the number of satellites expected today is
about an order of magnitude smaller than the number inferred by identifying
satellites with subhalos in high-resolution simulations of the dark matter. The
resulting satellite population has an abundance and a distribution of circular
velocities similar to those observed in the Local Group. We explore many other
properties of satellite galaxies, including their gas content, metallicity and
star formation rate, and find generally good agreement with available data. Our
model predicts the existence of many as yet undetected satellites in the Local
Group. We quantify their observability in terms of their apparent magnitude and
surface brightness and also in terms of their constituent stars. A
near-complete census of the Milky Way's satellites would require imaging to
V~20 and to a surface brightness fainter than 26 V-band magnitudes per square
arcsecond. Satellites with integrated luminosity V=15 should contain of order
100 stars brighter than B=26, with central stellar densities of a few tens per
square arcminute. Discovery of a large population of faint satellites would
provide a strong test of current models of galaxy formation.
