  Understanding quantum theory in terms of a geometric picture sounds great.
There are different approaches to this idea. Here we shall present a geometric
picture of quantum theory using the de-Broglie--Bohm causal interpretation of
quantum mechanics. We shall show that it is possible to understand the key
character of de-Broglie--Bohm theory, the quantum potential, as the conformal
degree of freedom of the space--time metric. In this way, gravity should give
the causal structure of the space--time, while quantum phenomena determines the
scale. Some toy models in terms of tensor and scalar--tensor theories will be
presented. Then a few essential physical aspects of the idea including the
effect on the black holes, the initial Big--Bang singularity and non locality
are investigated. We shall formulate a quantum equivalence principle according
to which gravitational effects can be removed by going to a freely falling
frame while quantum effects can be eliminated by choosing an appropriate scale.
And we shall see that the best framework for both quantum and gravity is Weyl
geometry. Then we shall show how one can get the de-Broglie--Bohm quantum
theory out of a Weyl covariant theory. Extension to the case of many particle
systems and spinning particles is discussed at the end.
