  This summary of several talks given in 1990-1993 discusses the problem of
asymptotic expansions of multiloop Feynman diagrams in masses and external
momenta - a central problem in perturbative quantum field theory. Basic
principles of the theory of asymptotic operation -- the most powerful tool for
that purpose -- are discussed. Its connection with the conventional methods is
explained (the BPHZ theory, the method of leading logarithmic approximation
etc.). The problem of non-euclidean asymptotic regimes is discussed as well as
ways of its solution.

High Energy Physics - Phenomenology