Book excerpt: Abstract: "In molecular dynamics applications there is a growing interest in including quantum effects for simulations of larger molecules. This paper is concerned with mixed quantum-classical models which are currently discussed: the so-called QCMD model with variants and the time- dependent Born-Oppenheimer approximation. All these models are known to approximate the full quantum dynamical evolution -- under different assumptions, however. We review the meaning of these assumptions and the scope of the approximation. In particular, we characterize those typical problematic situations where a mixed model might largely deviate from the full quantum evolution. One such situation of specific interest, a non- adiabatic excitation at certain energy level crossings, can promisingly be dealt with by a modification of the QCMD model that we suggest."