  We review some recent work on the existence and classification of extreme
black-hole-type solutions in N=8 supergravity. For the black holes considered
(those that are also solutions of N=4 supergravity and of the
Einstein-Maxwell-dilaton theory with coupling a) a complete classification is
achieved: the only possible values of a are \sqrt{3},1,1/\sqrt{3},0. Up to U
duality transformations there is only one solution for each of those values.
The exception is a=0 for which an additional extreme but non-supersymmetric
Reissner-Nordstr\"om black hole solution exists. We also study the so-called
massless black-hole solutions. We argue that they can be understood as
composite objects. At least one of the components would have ``negative mass''.
We also argue that these states, being annihilated by all the generators in the
supersymmetry algebra, could also constitute alternative vacua of the
supergravity theory.
