VIRTUES OF THE HAAG EXPANSION IN QUANTUM FIELD THEORY

I survey the use of the Haag expansion as a technique to solve quantum field theories. After an exposition of the asymptotic condition and the Haag expansion, I report the results of applying the Haag expansion to several quantum field theories, including galilean-invariant theories, matter at finite temperature (using the BCS model of superconductivity as an illustrative example), the Nambu--Jona-Lasinio model and the Schwinger model. I conclude with the outlook for further development of this method.