  Problems occurring in physically important non-trivial examples of loop
calculations are discussed.
  A procedure of deriving expansions of two-loop self-energy diagrams with
different masses is constructed. The cases of small and large external momentum
are considered. The coefficients of the expansions are calculated analytically.
Comparison with numerical results shows good agreement below the first
threshold and above the last threshold of the diagram.
  An approach to evaluating three- and four-point ladder diagrams with massless
internal particles and off-shell external momenta is presented. Exact results
for an arbitrary number of loops are obtained. Evaluating non-planar diagrams
is also discussed.
