The paper examines the substructure within galactic and cluster mass halos using numerical simulations in a hierarchical universe. The simulations show that galaxy clusters can be reproduced with a steep mass spectrum of thousands of substructure clumps, closely matching observations. However, dark matter substructure also survives on galactic scales, leading to the remarkable result that galaxy halos appear as scaled versions of galaxy clusters. The model predicts that the Milky Way's halo should contain about 500 satellites with circular velocities larger than Draco and Ursa-Minor, i.e., bound masses greater than $10^8 M_{\odot}$ and tidally limited sizes greater than kpc. These substructure clumps have orbits that take a large fraction of them through the stellar disk, leading to significant resonant and impulsive heating. The abundance and singular density profiles of these substructure clumps have important implications for the existence of old thin disks, cold stellar streams, gravitational lensing, and direct/direct detection experiments. The paper discusses the implications of these findings for the hierarchical model of galaxy formation and the challenges it poses for understanding the properties of galactic halos.The paper examines the substructure within galactic and cluster mass halos using numerical simulations in a hierarchical universe. The simulations show that galaxy clusters can be reproduced with a steep mass spectrum of thousands of substructure clumps, closely matching observations. However, dark matter substructure also survives on galactic scales, leading to the remarkable result that galaxy halos appear as scaled versions of galaxy clusters. The model predicts that the Milky Way's halo should contain about 500 satellites with circular velocities larger than Draco and Ursa-Minor, i.e., bound masses greater than $10^8 M_{\odot}$ and tidally limited sizes greater than kpc. These substructure clumps have orbits that take a large fraction of them through the stellar disk, leading to significant resonant and impulsive heating. The abundance and singular density profiles of these substructure clumps have important implications for the existence of old thin disks, cold stellar streams, gravitational lensing, and direct/direct detection experiments. The paper discusses the implications of these findings for the hierarchical model of galaxy formation and the challenges it poses for understanding the properties of galactic halos.