Timing of heavy ion collisions at SIS energies

A review of pion emission in heavy ion collisions is given and it is discus sed whether these spectra and the various particle yields can be interprete d within a thermal model with an emission at a unique temperature. A carefu l inspection of the high-energy pions reveals that they are emitted during the early phase of the collisions while pions with lower energies are emitt ed mainly later and over a long time interval.