Introduction -- The rigid diatomic rotor The spectra of diatomic and linear molecules -- The rotational spectra of none-linear molecules -- The determination of bond lengths and angles from microwave measurements -- The Stark effect in molecular spectra -- Inversion and restricted rotation -- Experimental methods of microwave spectroscopy -- Appendix 1. The Laplacian operator in spherical polar co-ordinates -- Appendix 2. Orthogonality and normalization of associated Legendre polynomials -- Appendix 3. The selection rules and matrix elements for electric dipole transitions of the diatomic rigid rotor -- Appendix 4. Solution of the wave equation for the symmetric top -- Appendix 5. Some properties of inertial dyadics -- Electric quadrupole moments.