  Recent N-Body simulations have shown that dIrrs evolve into dSphs owing to
the strong tidal perturbation of the Milky Way. Satellites whose dark matter
halos have a core or an NFW profile with a concentration $c < 5$ undergo severe
stripping even on low eccentricity orbits and their remnants have projected
stellar profiles that become flat at large radii as observed in some of the
dSphs. Satellites with higher central densities, as predicted by CDM models,
are more robust to tides and can reproduce the observations only on nearly
radial orbits. Accurate proper motions for LG dSphs will allow to test current
models of structure formation.
