A variety of quantum mechanical methods have been developed and applied to the study of highly energized "pre-reactive" species involved in chemical reactions. We have examined the character of transition states for both unimolecular systems and bimolecular collisions. For unimolecular reactions, we have studied vibrational predissociation of hydrogen peroxide. The nodal lines of the predissociative resonance states are found to bend toward the dissociative side. This character should be largely responsible for the dissociation of the molecule. To compute the dissociation rates, we have combined the complex coordinate method and the Lanczos algorithm. The complex Lanczos recursion method is found to be insufficient to produce well converged resonance widths for this large system due to round-off errors. For bimolecular collisions, we have computed the absorption spectra of the transition states of the reaction K + NaCl + hv → KCl + Na\sp\*. The absorption probabilities show a strong dependence on laser frequency. This dependence is well explained by Franck-Condon calculations. By contrast, a linear curve crossing model is quantitatively incorrect. After carefully examining the excited wave packet dynamics and the time evolution of the transition probabilities, we believe the excitation process is not localized to the crossings of the field-dressed potential curves. We have also studied the effects of overall molecular rotation on the vibrational dynamics and unimolecular reaction rates. For a simple collinear triatomic model, the dissociation rates are uniformly increased as a function of angular momentum J, generally in a manner close to J\sp2. The reaction rates could be changed by a factor of three for some predissociative states, while remain almost unchanged for some other ones. The differences in the J-dependency correlate well with the existence of Fermi resonance conditions. The rotational effects are further investigated using a more realistic three dimensional model for HCN/HNC isomerization. We have developed a parameter dependent basis set for the study of this particular system. The importance of overall molecular rotation is confirmed in this study. However, the overall rotation is found to have non-uniform effects for different initial states.