Bio-molecules have a large number of vibrational degrees of freedom, which are more or less strongly coupled to transitions between different electronic states. In this chapter, we discuss the vibronic states on the basis of the Born–Oppenheimer separation and the remaining nonadiabatic coupling of the adiabatic states. The nuclear motion is approximated by independent harmonic normal modes. Matrix elements of the nuclear gradient operator are evaluated in the model of parallel displaced harmonic oscillators. Finally, we discuss mixing of the Born-Oppenheimer states due to nonadiabatical coupling and the damping approximation for finite lifetimes.