We have developed a method for describing the reaction dynamics of a polyatomic molecule in intense laser fields. First, the dynamical behavior of H 2 + and H 2 in near-infrared, intense laser fields (I > 10 13 W cm -2 and λ > 700 nm) was examined; accurate evaluation of the electronic and nuclear wave packet was achieved by the dual transformation method that we developed. Using "field-following" time-dependent adiabatic states defined as eigenfunctions of the "instantaneous" electronic Hamiltonian, we have clarified the dynamics of bound electrons, ionization processes, Coulomb explosion processes, and molecular vibrations of H 2 + and H 2. The analyses indicate that the multielectron dynamics and nuclear dynamics of polyatomic molecules in intense fields can be described by using the potential surfaces of time-dependent adiabatic states and the nonadiabatic coupling elements between those states. To obtain time-dependent adiabatic states of a molecule, one can diagonalize the electronic Hamiltonian including the interaction with the instantaneous laser electric field by ab initio molecular orbital (MO) methods. The time-dependent adiabatic potentials obtained are used to evaluate the multichannel nuclear dynamics until the next ionization process. We have applied the time-dependent adiabatic state approach to reveal the characteristic features of the dynamics of structural deformations of CO 2 and its cations in a near-infrared intense laser field. The experimentally observed stretched and bent structure of CO 2 3+ just before Coulomb explosions originates from the structural deformation of CO 2 2+. We also revealed the mechanism of the experimentally observed bond dissociation of C 2H 5OH; we found that the relative probability of C-O bond cleavage to that of C-C bond cleavage becomes smaller with decreases in the pulse length. This example clearly shows that field-induced nonadiabatic transitions play a decisive role in the reaction dynamics of molecules in an intense laser field.