mp_arc@math.utexas.edu, \# 93-318 Author: G. Gallavotti Title: Perturbation theory Abstract: Notes in margin to the {\it Mathematical Physics towards the XXI century} conference, University of Negev, Beer Sheva, 14--19 march, 1993. In "Mathematical physics towards the XXI century", p. 275--294, ed. R. Sen, A. Gersten, Ben Gurion University Press, Ber Sheva, 1994. Keywords: Perturbation theory Giovanni Gallavotti Fisica, Universita' di Roma La Sapienza, P.le Moro 2, 00185, Roma, Italia. e-mail giovanni@ipparco.roma1.infn.it tel. 6-49914370, fax 6-4957697 Home page: http://www.math.rutgers.edu/~giovanni