  The measurement and interpretation of isotopic yield ratios in heavy ion
reactions at intermediate and high energies are discussed and the usefulness of
these observables for establishing equilibrium properties and for determining
thermodynamic parameters is illustrated. The examples are mainly taken from
work performed with lighter projectiles at intermediate and high energies and
from studies of spectator reactions at relativistic energies. As an
application, the caloric curve of nuclei, as derived for Au on Au collisions,
is introduced and discussed.
