  Milgrom has proposed that the appearance of discrepancies between the
Newtonian dynamical mass and the directly observable mass in astronomical
systems could be due to a breakdown of Newtonian dynamics in the limit of low
accelerations rather than the presence of unseen matter. Milgrom's hypothesis,
modified Newtonian dynamics or MOND, has been remarkably successful in
explaining systematic properties of spiral and elliptical galaxies and
predicting in detail the observed rotation curves of spiral galaxies with only
one additional parameter-- a critical acceleration which is on the order of the
cosmologically interesting value of $cH_o$. Here I review the empirical
successes of this idea and discuss its possible extention to cosmology and
structure formation.
