  I describe the MOND paradigm, which posits a departure from standard physics
below a certain acceleration scale. This acceleration as deduced from the
dynamics in galaxies is found mysteriously to agree with the cosmic
acceleration scales defined by the present day expansion rate and by the
density of `dark energy'. I put special emphasis on phenomenology and on
critical comparison with the competing paradigm based on classical dynamics
plus cold dark matter. I also describe briefly nonrelativistic and relativistic
MOND theories.
