  We study general black-hole solutions of the low-energy string effective
action in arbitrary dimensions using a general metric that can describe them
all in a unified way both in the extreme and non-extreme cases. We calculate
the mass, temperature and entropy and study which relations amongst the charges
and the mass lead to extremality. We find that the temperature always vanishes
in the extreme limit and we find that, for a set of n charges (no further
reducible by duality) there are 2^{(n-1)} combinations of the charges that
imply extremality. Not all of these combinations can be central charge
eigenvalues and, thus, there are in general extreme black holes which are not
supersymmetric (or ``BPS-saturated''). In the N=8 supergravity case we argue
that the existence of roughly as many supersymmetric and non-supersymmetric
extreme black holes suggests the existence of an underlying twelve-dimensional
structure.
