Quantum cosmology: How to interpret and obtain results

We argue that the Copenhagen interpretation of quantum mechanics cannot be applied to quantum cosmology. Among the alternative interpretations, we cho ose to apply the Bohm-de Broglie interpretation of quantum mechanics to can onical quantum cosmology. For minisuperspace models, ive show that there is no problem of time in this interpretation, and that quantum effects can av oid the initial singularity, create inflation and isotropize the Universe. For the general case, it is shown that, irrespective of any regularization or choice of factor ordering of the Wheeler-DeWitt equation, the unique rel evant quantum effect which does not break spacetime is the change of its si gnature from lorentzian to euclidean. The other quantum effects are either trivial or break the four-geometry of spacetime. A Bohm-de Broglie picture of a quantum geometrodynamics is constructed, which allows the investigatio n of these latter structures.