  Everett's interpretation of quantum mechanics was proposed to avoid problems
inherent in the prevailing interpretational frame. It assumes that quantum
mechanics can be applied to any system and that the state vector always evolves
unitarily. It then claims that whenever an observable is measured, all possible
results of the measurement exist. This notion of multiplicity has been
understood in different ways by proponents of Everett's theory. In fact the
spectrum of opinions on various ontological questions raised by Everett's
approach is rather large, as we attempt to document in this critical review. We
conclude that much remains to be done to clarify and specify Everett's
approach.
