  An overview is given of the methods for treating complicated problems without
small parameters, when the standard perturbation theory based on the existence
of small parameters becomes useless. Such complicated problems are typical of
quantum physics, many-body physics, physics of complex systems, and various
aspects of applied physics and applied mathematics. A general approach for
dealing with such problems has been developed, called Self-Similar
Approximation Theory. A concise survey of the main ideas of this approach is
presented, with the emphasis on the basic notion of group self-similarity. The
techniques are illustrated by examples from quantum field theory.
