  The term Bethe Ansatz stands for a multitude of methods in the theory of
integrable models in statistical mechanics and quantum field theory that were
designed to study the spectra, the thermodynamic properties and the correlation
functions of these models non-perturbatively. This essay attempts to a give a
brief overview of some of these methods and their development, mostly based on
the example of the Heisenberg model and the corresponding six-vertex model.
