Semiclassical methods in finite temperature field theory

We study some semiclassical methods and non-perturbative phenomena in finite temperature field theory. A discussion of theoretical concepts used to study quantum tunneling in systems with many degrees of freedom is given. A semiclassical approximation for the probability distribution function rho(phi) = in field theory is presented and is shown to yield the exact distribution functions for a free scalar field theory and a pure Abelian gauge theory. A Hartree self-consistent field approximation for the distribution function is also discussed. Quantitative calculations of baryon and lepton number violation by electroweak instantons at zero and finite temperature are reported