The rotational spectrum of jet-cooled hafnium dioxide obtained by laser ablation of a solid ceramic rod has been investigated by Fourier transform microwave spectroscopy in the frequency range 8 Hz to 28 GHz. Rotational transitions from the ground and several excited vibrational states of the lowest vibrational mode of the molecule have been assigned. Centrifugal distortion effects are noticeable even for the lowest-J transitions. Very large quadrupole coupling effects for the isotopomers with nuclear quadrupole moments (179Hf (I=9/2) and 177Hf (I=7/2)) have been accounted for using the diagonal elements of the nuclear quadrupole coupling tensor. A ground-state effective C2v geometry has been obtained for HfO2, yielding ro(Hf-O) = 1.7764(4) and (O-Hf-O) = 107.51(1). The electric dipole moment has been determined for 180HfO2 from Stark-effect measurements, thus = 26.42(3) (10 to 30) C m [7.92(1) D]. Ab initio calculations using density functional theory and relativistic core potentials are in very satisfactory agreement with the experimental results. Finally, in the course of this investigation the rotational spectrum of the diatomic molecule HfO has also been reexamined, and new results for vibrational satellites (up to v=18 in some cases) of the J = 1 to 0 rotational transition are reported.