  We describe representations of groupoid C*-algebras on Hilbert modules over
arbitrary C*-algebras by a universal property. For Hilbert space
representations, our universal property is equivalent to Renault's
Integration-Disintegration Theorem. For a locally compact group, it is related
to the automatic continuity of measurable group representations. It implies
known descriptions of groupoid C*-algebras as crossed products for \'etale
groupoids and transformation groupoids of group actions on spaces.
