Pure rotational spectra of 1,1-Difluoroethylene (1,1-DFE) in the vibrational ground state (GS) and in excited vibrational states (EVSs) have been investigated by double resonance modulation (DRM) microwave spectroscopy in the range from 12 GHz to 40 GHz, and by millimeterwave spectroscopy in the range from 90 to 260 GHz. In addition to the GS spectrum, rotational transitions have been observed for the EVSs ν 10 = 1, and 2, ν 12 = 1, ν 6 = 1, ν 11 = 1, ν 4 = 1, and ν 9 = 1. Along with other IR bands up to 1400cm -1 , the fundamental vibrational band ν 10 = 1 ← 0 has been observed by FT-IR spectroscopy and has been analysed to yield ν 10 0 = 436.8851 cm -1 . Rotational constants and quartic distortion constants were deduced for all observed states. In addition, sextic distortion constants were obtained for the GS and for the EVSs ν 10 = 1, 2, ν 6 = 1, and ν 12 = 1 by computer fits of combined microwave and millimeterwave data to Watson's A-reduced Hamiltonian in I r representation. The S-reduced Hamiltonian in III r representation may be considered more appropriate for the calculation or the fitting of the transitions because 1,1-DFE is a near oblate asymmetric top. Thus, for comparison, results are given for both the A- and the S-reduction.