The results of experimental and theoretical studies of the influence of large-amplitude motions of nuclei on the IR and Raman spectra, electrooptical parameters, and structural features of molecular systems in various states of aggregation are generalized. The mechanisms of intramolecular rearrangements in these systems are considered. The potential of the finite element method for the description of large-amplitude motions of arbitrary form is demonstrated. A method for estimation of the characteristic time of intramolecular rearrangements is proposed.