Two sketches of a galaxy with its associated dark matter halo. On the left is a simple picture, of a smooth dark matter distribution -- possibly mildly oblate or triaxial. On the right is an n-body simulation of a more realistic halo, with significant substructure. It is still unclear whether the substructure is present in real halos -- certainly, if it is, then not all sub-clumps contain dwarf galaxies. The optical galaxy has been superimposed at roughly the correct scale.