The satellite galaxies of the Milky Way and Andromeda

The satellite galaxies of the Local Group provide us with an important probe of galaxy formation, evolution, and cosmology. The two large spirals that dominate this group -- the Milky Way and Andromeda -- are each host to tens of satellites, ranging in stellar mass from $M_*=3\times10^9\,{\rm M_\odot}$ down to as little as $M_*\sim1000\,{\rm M_\odot}$. In this review, we (1) provide an overview of the known satellite population of the Milky Way and Andromeda, including how they are discovered and their observed properties; (2) discuss their importance in understanding the nature of dark matter, star formation in the early Universe, the assembly histories of their massive hosts, and the impact of reionisation on the lowest mass galaxies; and (3) highlight the coming revolution and challenges of this field as new observatories and facilities come online. In the coming decades, the study of Local Group satellites should allow us to place competitive constraints on both dark matter and galaxy evolution.