  If nature is described by string theory, and if the compactification radius
is large (as suggested by the unification of couplings), then the theory is in
a regime best described by the low energy limit of $M$-theory. We discuss some
phenomenological aspects of this view. The scale at which conventional quantum
field theory breaks down is of order the unification scale and consequently
(approximate) discrete symmetries are essential to prevent proton decay. There
are one or more light axions, one of which solves the strong CP problem.
Modular cosmology is still problematic but much more complex than in
perturbative string vacua. We also consider a range of more theoretical issues,
focusing particularly on the question of stabilizing the moduli. We give a
simple, weak coupling derivation of Witten's expression for the dependence of
the coupling constants on the eleven dimensional radius. We discuss the
criteria for the validity of the long wavelength analysis and find that the
"real world" seems to sit just where this analysis is breaking down. On the
other hand, residual constraints from N=2 supersymmetry make it difficult to
see how the moduli can be stabilized while at the same time yielding a large
hierarchy.
