The most metal-poor stars provide important constraints on a number of problems, including the modeling of the early stages of galaxy assembly, the nature of first stars and the reionization. I present here numerical results on the distribution of metal-poor stars in stellar halos formed in a ΛCDM cosmology. I then discuss the results in light of current observational data of very metal-poor stars in the halos of Milky Way, Andromeda and neighboring dwarf galaxies.