  We study the Torelli morphism from the moduli space of stable curves to the
moduli space of principally polarized stable semi-abelic pairs. We give two
characterizations of its fibers, describe its injectivity locus, and give a
sharp upper bound on the cardinality of the finite fibers. We also bound the
dimension of the infinite fibers. In particular, we obtain that the
compactified Torelli map is injective away from curve with separating nodes if
and only if the genus is at most 4.
