  We investigate compactifications of M-theory from $11\to 5\to 4$ dimensions
and discuss geometrical properties of 4-d moduli fields related to the
structure of 5-d theory. We study supersymmetry breaking by compactification of
the fifth dimension and find that an universal superpotential is generated for
the axion-dilaton superfield $S$. The resulting theory has a vacuum with
$<S>=1$, zero cosmological constant and a gravitino mass depending on the fifth
radius as $m_{3/2} \sim R_5^{-2}/M_{Pl}$. We discuss phenomenological aspects
of this scenario, mainly the string unification and the decompactification
problem.
