  Modified Newtonian dynamics (MOND) is an empirically motivated modification
of Newtonian gravity or inertia suggested by Milgrom as an alternative to
cosmic dark matter. The basic idea is that at accelerations below a0 ~ 10^{-8}
cm/s^2 ~ cH0/6 the effective gravitational attraction approaches sqrt{gN*a0}
where gN is the usual Newtonian acceleration. This simple algorithm yields flat
rotation curves for spiral galaxies and a mass-rotation velocity relation of
the form M ~ V^4 that forms the basis for the observed luminosity-rotation
velocity relation-- the Tully-Fisher law. We review the phenomenological
success of MOND on scales ranging from dwarf spheroidal galaxies to
superclusters, and demonstrate that the evidence for dark matter can be equally
well interpreted as evidence for MOND. We discuss the possible physical basis
for an acceleration-based modification of Newtonian dynamics as well as the
extension of MOND to cosmology and structure formation.
