We discuss the observation of coherent nuclear oscillations in chemical and biological systems subsequent to excitation with an ultrashort laser pulse. The experimentally observed coherent signals arising from rapid, photon initiated, chemical reactions are considered separately from vibrational coherences directly excited by the (electric) field interventions of the Light pulse. In order to treat the chemical reaction, a modified Landau-Zener theory is developed that is quasi-classical along the traditional reaction coordinate but fully quantum mechanical in all other degrees of freedom. In this work, we explore explicitly the effects of electronic state coupling, electron-nuclear coupling, reaction velocity, vibrational damping and temperature on the product electronic state population development and on the coherent oscillations of the nuclear coordinates. Recent experimental studies of the ligand photolysis reaction in heme proteins are presented as examples. (C) 1997 American Institute of Physics.