The interaction between electrons and phonons in molecules is investigated using a formulation in which the total molecular Hamiltonian is subjected to two canonical transformations. The first of these transformations, which we term the normal coordinatetransformation, passes from a crude representation, which we term the clamped nuclei representation, in terms of basis functions defined with respect to some fixed geometry of the nuclei, R0, to a representation in terms of basis functions depending onR. This transformation mixes electronic and vibrational motions and leads to a simple formulae for adiabatic corrections. The second transformation, termed the momentum transformation, leads to a non-adiabatic (or diabatic) representation.