  In this series of lectures we present the universal method based on the use
of the functional integrals to derive the bound state equations for different
two-body systems in elementary particle physics as well as in condensed matter
theory: the relativistic Nambu-Jona-Lasinio equation and the relativistic
Bethe-Salpeter equation in elementary particle physics, the Bethe-Salpeter
equation for the bipolaritons. The bound state equation for the three-body
systems is also established.
