  A general account of MOND theory is given. I start with the basic tenets of
MOND, which posit departure from standard dynamics in the limit of low
acceleration -- below an acceleration constant a0 -- where dynamics become
scale invariant. I list some of the salient predictions of these tenets. The
special role of a0 and its significance are then discussed. In particular, I
stress its coincidence with cosmologically relevant accelerations. The
deep-MOND limit and the consequences of its scale invariance are considered in
some detail.
  General aspects of MOND theories are then described, after which I list
briefly presently known theories, both nonrelativistic and relativistic. Most
full-fledged theories modify the gravitational action, hinge on a0, introduce
an interpolating function between the low and high accelerations, and obey MOND
requirements in the opposite limits. These theories have much heuristic value
as proofs of various concepts (e.g., that covariant MOND theories can be
written with correct gravitational lensing). But, probably, they are, at best,
effective theories of limited applicability.
  I then outline several other promising approaches to constructing MOND
theories that strive to obtain MOND as an effective theory from deeper
concepts, for example, by modifying inertia and/or gravity as a result of
interactions with some omnipresent agent.
  Some theories do enjoy a natural appearance of a cosmological-constant-like
contribution that, furthermore, exhibits the observed connection with a0.
However, none were shown to address fully the mass discrepancies in cosmology
and structure formation that are otherwise explained by cosmological dark
matter.
  We have no clues as to whether and how MOND aspects enter non-gravitational
phenomena, but I discuss briefly some possibilities.
