  We begin by discussing ``What exists?'', i.e. ontology, in Classical Physics
which provided a description of physical phenomena at the macroscopic level.
The microworld however necessitates a introduction of Quantum ideas for its
understanding. It is almost certain that the world is quantum mechanical at
both microscopic as well as at macroscopic level. The problem of ontology of a
Quantum world is a difficult one. It also depends on which interpretation is
used. We first discuss some interpretations in which Quantum Mechanics does not
provide a complete framework but has to be supplemented by extra ingredients
e.g. (i) Copenhagen group of interpretations associated with the names of Niels
Bohr, Heisenberg, von-Neumann, and (ii) de-Broglie-Bohm interpretations. We
then look at some interpretations in which Quantum mechanics is supposed to
provide the entire framework such as (i) Everett-deWitt many world, (ii)
quantum histories interpretations. We conclude with some remarks on the
rigidity of the formalism of quantum mechanics, which is sharp contrast to it's
ontological fluidity.
