  Any viable theory of modified Newtonian dynamcs (MOND) as modified gravity is
likely to require fields in addition to the usual tensor field of General
Relativity. For such theories the MOND phenomenology emerges from an effective
fifth force probably associated with a scalar field. Here I consider the
constraints imposed upon such theories by solar system phenomenology, primarily
by the absence of significant deviations from inverse square attraction in the
inner solar system as well as detectable local preferred frame effects. The
current examples of multi-field theories can be constructed to satisfy these
constraints and such theories lead inevitably to an anomalous non
inverse-square force in the outer solar system.
