Canonical quantization approach to 2d gravity coupled to c<1 matter

We show that all important features of 2d gravity coupled to $c<1$ matter can be easily understood from the canonical quantization approach a la Dirac. Furthermore, we construct a canonical transformation which maps the theory into a free-field form, i.e. the constraints become free-field Virasoro generators with background charges. This implies the gauge independence of the David-Distler-Kawai results, and also proves the free-field assumption which was used for obtaining the spectrum of the theory in the conformal gauge. A discussion of the unitarity of the physical spectrum is presented and we point out that the scalar products of the discrete states are not well defined in the standard Fock space framework.