Form Factors in Affine Toda Field Theories

I briefly review the properties of classical affine Toda field theories and indicate how some of this features survive in the quantum theory on-shell. I demonstrate how this knowledge can be extended off-shell, i.e. how to compute correlation functions for completely integrable models via the form factor approach. For the latter I present an axiomatic system and two explicit computation (the Sinh-Gordon theory and the Bullough-Dodd model) which provide a consistent solution of it. (Talk presented at the VII Andr\'e Swieca Summer School, Campos do Jord\~ao, Brasil, 1993)