  A survey of various mechanisms for particle emission in hydrodynamics is
presented. First, in the case of sudden freeze out, the problem of negative
contributions in the Cooper-Frye formula and ways out are presented. Then the
separate chemical and thermal freeze out scenario is described and the
necessity of its inclusion in a hydrodynamical code is discussed. Finally, we
show how to formulate continuous particle emission in hydrodynamics and discuss
extensively its consistency with data. We point out in various cases that the
interpretation of data is quite influenced by the choice of the particle
emission mechanism.
