  It has long been known that Newtonian dynamics applied to the visible matter
in galaxies and clusters does not correctly describe the dynamics of those
systems. While this is generally taken as evidence for dark matter it is in
principle possible that instead Newtonian dynamics (and with it General
Relativity) breaks down in these systems. Indeed there have been a number of
proposals as to how standard gravitational dynamics might be modified so as to
correctly explain galactic dynamics without dark matter. I will review this
general idea (but focus on ``MOdified Newtonian Dynamics'', or ``MOND''), and
discuss a number of ways alternatives to dark matter can be tested and (in many
cases) ruled out.
