  We describe our recent attempts to model substructure in dark matter halos
down to very small masses, using a semi-analytic model of halo formation. The
results suggest that numerical simulations of halo formation may still be
missing substructure in the central regions of halos due to purely numerical
effects. If confirmed, this central 'overmerging' problem will have important
consequences for the interpretation of lensing measurements of substructure. We
also show that the spatial distribution of subhalos relative to the satellite
companions of the Milky Way rules out at least one simple model of how dwarf
galaxies form in low-mass halos.
