We discuss ab initio molecular orbital studies of vibronic spectra and some dynamic properties of polyatomic molecules, including the rate of internal conversion, based on calculations of potential energy surfaces for the ground and excited states. Ab initio molecular orbital (MO) calculations using such advanced and accurate methods as multiconfigurational SCF (CASSCF), internally contracted multireference configuration interaction (MRCI), equation-of-motion coupled cluster (EOM-CCSD), etc., provide, reliable information on  the geometries, the vibrational frequencies and the vertical and adiabatic excitation energies of excited electronic states for various polyatomic molecules. Vibronic spectra are determined through calculations of vibrational overlap integrals and Franck-Condon factors. The derivation of the formula for the vibrational overlap integrals for the case of displaced, distorted and rotated normal coordinates is presented. The calculated wavefunctions are used to determine matrix elements for vibronic coupling between different electronic states. The coupling elements are utilized in the calculations of intensities of forbidden transitions in vibronic spectra and for computations of the rate of internal conversion, relevant to photodissociation processes. We discuss applications of this ab initio/spectroscopy/dynamics approach to small polyatomic molecules and radicals, including C₂H₄, CH₄, C₂H₃, CH₃, and C₃H₂.