Adiabatic and non-adiabatic evolution of wave packets and applications to initial value representations

We review some recent results obtained for the time evolution of wave packets for systems of equations of pseudo-differential type, including Schr{ö}dinger ones, and discuss their application to the approximation of the associated unitary propagator. We start with scalar equations, propagation of coherent states, and applications to the Herman-Kluk approximation. Then we discuss the extension of these results to systems with eigenvalues of constant multiplicity or with smooth crossings.