Strange messages: chemical and thermal freeze-out in nuclear collisions

Thermal models are commonly used to interpret heavy-ion data on particle yi elds and spectra and to extract the conditions of chemical and thermal free ze-out in heavy-ion collisions. I discuss the usefulness and limitations of such thermal model analyses and review the experimental and theoretical ev idence for thermalization in nuclear collisions. The crucial role of correl ating strangeness production data with single particle spectra and two-part icle correlation measurements is pointed out. A consistent dynamical pictur e for the heavy-ion data from the CERN SPS involves an initial prehadronic stage with deconfined colour and with an appreciable isotropic pressure com ponent. This requires an early onset of thermalization.