The history of the Galaxy is imprinted in the kinematics and chemical
properties of the stars in the stellar halo. Their study allows us
partially to reconstruct the Galactic past because the time required
for stars in the halo to exchange their energies and momenta is very
long compared with the age of the Galaxy. This field has been
revolutionised in recent years by data from the Sloan Digital Sky
Survey Data, which has revealed a super-abundance of substructure.
There are ghostly streams from disrupting and coalescing globular
clusters, analogues of meteor streams along old cometary paths in the
Solar system. There are ultrafaint phantom galaxies composed of old
and faint stars, so puny that the entire galaxy is outshone by a
single red giant star like Betelgeuse. Above all, dominating the
Galactic halo, there is the magnificent and puzzling double arch of
the Sagittarius stream criss-crossing the whole sky. I will review the
discoveries and assess their present status.