  The two-dimensional Ising model is the simplest model of statistical
mechanics exhibiting a second order phase transition. While in absence of
magnetic field it is known to be solvable on the lattice since Onsager's work
of the forties, exact results for the magnetic case have been missing until the
late eighties, when A.Zamolodchikov solved the model in a field at the critical
temperature, directly in the scaling limit, within the framework of integrable
quantum field theory. In this article we review this field theoretical approach
to the Ising universality class, with particular attention to the results
obtained starting from Zamolodchikov's scattering solution and to their
comparison with the numerical estimates on the lattice. The topics discussed
include scattering theory, form factors, correlation functions, universal
amplitude ratios and perturbations around integrable directions. Although we
restrict our discussion to the Ising model, the emphasis is on the general
methods of integrable quantum field theory which can be used in the study of
all universality classes of critical behaviour in two dimensions.
