Alternatives to Dark Matter (?)

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.