Non-perturbative methods
  The current theoretical understanding of processes involving many weakly interacting bosons in the Standard Model and in model theories is discussed. In particular, such processes are associated with the baryon and lepton number violation in the Standard Model. The most interesting domain where the multiplicity of bosons is larger than the inverse of small coupling constant is beyond the scope of perturbation theory and requires a non-perturbative analysis. 
M.B. Voloshin