  Planet-disk interaction predicts a change in the orbital elements of an
embedded planet. Through linear and fully hydrodynamical studies it has been
found that migration is typically directed inwards. Hence, this migration
process gives natural explanation for the presence of the 'hot' planets
orbiting close to the parent star, and it plays a mayor role in explaining the
formation of resonant planetary systems.
  However, standard migration models for locally isothermal disks indicate a
too rapid inward migration for small mass planets, and a large number of
massive planets are found very far away from the star. Recent studies,
including more complete disk physics, have opened up new paths to slow down or
even reverse migration. The new findings on migration are discussed and
connected to the observational properties of planetary systems.
