Heidelberg Joint Astronomical Colloquium

N-body simulations can exhibit rich substructure in the Galactic disk and halo that could be revealed from ongoing and future surveys. A simulated disc can show clumps and gaps in the local velocity distributions all over the galaxy. Gaps in the velocity distribution are associated with both discontinuities in the spiral arm morphology, such as pitch angle variations or the appearance of armlets, and Lindblad resonances with the patterns of spiral and bar waves. These features are ubiquitous because there are multiple coupled waves traveling through the disk. Simulations of tidal streams from disrupted clusters and dwarf galaxies suggest that they are Jeans unstable. The dark matter subhalo distribution not only provides seed perturbations for the instability but sets the fastest growing unstable wavelength, providing a link between the spacing of small clumps in a tidal stream and the dark matter distribution. Implications for interpretations of observations of the Milky Way will be discussed.