  We give an explicit construction of a positive-definite invariant
inner-product for the Klein-Gordon fields, thus solving the old problem of the
probability interpretation of Klein-Gordon fields without having to restrict to
the subspaces of the positive-frequency solutions. Our method has a much wider
domain of application and may be used to obtain the most general invariant
inner-product on the solution space of a broad class of Klein-Gordon type
evolution equations. We explore its consequences for the solutions of the
Wheeler-DeWitt equation associated with the FRW-massive-real-scalar-field
models.
