We use numerical simulations to examine the substructure within galactic and cluster mass halos that form within a hierarchical universe. Clusters are easily reproduced with a steep mass spectrum of thousands of substructure clumps that closely matches observations. However, the survival of dark matter substructure also occurs on galactic scales, leading to the remarkable result that galaxy halos appear as scaled versions of galaxy clusters. The model predicts that the virialised extent of the Milky Way's halo should contain about 500 satellites with circular velocities larger than Draco and Ursa-Minor i.e. bound masses > 10^8Mo and tidally limited sizes > kpc. The substructure clumps are on orbits that take a large fraction of them through the stellar disk leading to significant resonant and impulsive heating. Their abundance and singular density profiles has important implications for the existence of old thin disks, cold stellar streams, gravitational lensing and indirect/direct detection experiments.

Astrophysical Journal Letters. 4 pages, latex. Simulation images and movies at http://star-www.dur.ac.uk:80/~moore/