  Although the modified dynamics (MOND) proposed by Milgrom successfully
accounts for the systematics of galaxy rotation curves and cluster dynamics
without invoking dark matter, the idea remains a largely ad hoc modification of
Newtonian dynamics with no basis in deeper theory. Non-standard scalar-tensor
theories have been suggested as a theoretical basis for MOND; however, any such
theory with the usual conformal relation between the Einstein and physical
metrics fails to predict the degree of light deflection observed in distant
clusters of galaxies. In the present paper, I demonstrate that one can write
down a framework for scalar-tensor theories which predict the MOND
phenomenology for the low velocity (v<<c) dynamics of galaxies and clusters of
galaxies and which are consistent with observations of extragalactic
gravitational lenses provided that one drops the requirement of the Lorentz
Invariance of gravitational dynamics. This leads to ``preferred frame''
theories characterized by a non-conformal relation between the two metrics. I
describe a toy theory in which the local environment (the solar system, binary
pulsars) is protected from detectable preferred frame effects by the very same
non-standard (aquadratic) scalar Lagrangian which gives rise to the MOND
phenomenology. Although this particular theory is also contrived, it represents
a limiting case for two-field theories of MOND, and is consistent with a wide
range of gravitational phenomena.
